AN-154 - National Semiconductor

TL/C/7213

1.3V IC Flasher, Oscillator, Trigger or Alarm AN-154

National Semiconductor

Application Note 154

December 1975

1.3V IC Flasher, Oscillator,

Trigger or Alarm

INTRODUCTION

Most linear integrated circuits are designed to operate with

power supplies of 4.5 to 40V. Practically no battery/portable

equipment is provided with indicator lights due to unaccept-

able power drain. Even LEDs (solid state lamps) won’t light

from a 1.5V battery, and drain the common 9V radio battery

in a few hours.

The LM3909 changes all this. Obtaining long life from a sin-

gle 1.5V cell, it opens a whole new area of applications for

linear integrated circuits. Sufficient voltage for flashing a

light emitting diode is generated with cell voltage down to

1.1V. In such low duty cycle applications batteries will last

for months to years of continuous operation. Such flasher

circuits then become practical for marking location of flash-

lights, emergency equipment, and boat mooring floats in the

dark.

The LM3909 is simple in design, easy to use, and includes

extra resistors to minimize external circuitry and the size of

the completed flasher or oscillator.

CIRCUIT OPERATION

The circuit below in

Figure A

is the LM3909 connected as

the simplest type of oscillator. Ignoring the capacitor for a

moment, and assuming 1.5V on pin 5, current will flow in the

3k and 6k timing resistors through the emitter of Q1. This

current will be amplified by about 3 by Q2and passed to the

base of Q3.Q

3will then conduct, pulling down on the base

of Q4and hence the base of Q1. This is a negative feedback

since it will reduce timing resistor current and current to the

power transistor’s base until a balance is reached. This will

occur with the collector of Q3at about 0.5V, the base of Q4

at about 1V, and a very small voltage from pin 8 to ground.

The difference between these two voltages is the base-

emitter drop of Q1and 2/3 the base-emitter drop of Q4as

set by the high resistance divider from its base to emitter.

Note that negative feedback

voltage

is attenuated by at

least 2 due to the divider of two 400Xresistors. Now con-

sidering the capacitor, its positive feedback is initially unity.

Therefore the DC bias condition and the temporary excess

positive feedback conditions are met and the circuit must

oscillate.

The waveform at pin 8 of the above oscillator is shown be-

low. The waveform at pin 2, the power transistor collector, is

almost a rectangle. It extends from a saturation voltage of

0.1V or less to within about 0.1V of the supply voltage. The

‘‘on’’ period of course coincides with the negative pulses at

pin 8. Other circuit voltages can easily be inferred from the

two waveforms in

Figure B

TL/C/7213–1

FIGURE A

TL/C/7213–2

FIGURE B

C1995 National Semiconductor Corporation RRD-B30M105/Printed in U. S. A.

The simplicity of LED and incandescent pilot lamp flashers

is illustrated below in

Figure 1.

In the LED flasher, the

LM3909 uses the single capacitor for both timing and volt-

age boosting.

The LM3909, although designed as a LED flasher, is ideal

for other applications such as high current, trigger pulse for

SCRs and ‘‘Triacs.’’ The frequency of oscillation adjusts

from under 1 Hz to hundreds of kHz. Waveshape can be set

from pulses a few ms wide to approximately a square wave.

Thus the LM3909 can perform as a sound effects generator,

an audible alarm, or audible continuity checker. Finally it can

be a radio (detector/amplifier), low power one-way inter-

com, two-way telegraph set, or part of a ‘‘mini-strobe’’ light

flashing up to 7 times per second.

Operating with only a 1.5V battery as a supply gives the

LM3909 several rather unique characteristics. First,

known connection can cause immediate destruction of the

IC. Its internal feedback loop insures self-starting of properly

loaded oscillator circuits. Experimenters can safely explore

the possibilities of the LM3909 as an AC amplifier, one-shot,

latch circuit, resistance limit detector, multi-tone oscillator,

heat detector, or high frequency oscillator.

With the accent on the practical, a brief circuit description

will be given followed by circuits in the following application

areas:

Flasher & Indicator Applications

Audio & Oscillator Applications

Trigger & Other Applications

For those who want to modify or design their own circuits

using the LM3909, application hints will be covered near the

end of this note.

CIRCUIT DESCRIPTION

The circuit of

Figure 2

again shows the typical 1.5V LED

flasher, but with the internal circuitry of the IC illustrated.

The flasher achieves minimum power usage in two ways.

Operated as above, the LED receives current only about

1% of the time. The rest of the time, all transistors but Q4

are off. The 20k resistor from Q4’s emitter to supply-com-

mon draws only about 50 mA. The 300 mF capacitor is

charged through the two 400Xresistors connected to pin 5

and through the 3k resistor connected to pin 1 of the circuit.

1.5V Flasher Incandescent Bulb Flasher

TL/C/7213–3

Note: Nominal Flash Rate: 1 Hz. Note: Flash Rate: 1.5 Hz.

FIGURE 1. Two Simple Flashers

TL/C/7213–4

FIGURE 2. Circuit Operation

Transistors Q1through Q3remain off until the capacitor be-

comes charged to about 1V. This voltage is determined by

the junction drop of Q4, its base-emitter voltage divider, and

the junction drop of Q1. When voltage at pin 1 becomes a

volt more negative than that at pin 5 (supply positive termi-

nal) Q1begins to conduct. This then turns on Q2and Q3.

The LM3909 then supplies a pulse of high current to the

LED. Current amplification of Q2and Q3is between 200 and

1000. Q3can handle over 100 mA and rapidly pulls pin 2

close to supply common (pin 4). Since the capacitor is

charged, its other terminal at pin 1 goes

below

the supply

common. The voltage at the LED is then higher than battery

voltage, and the 12Xresistor between pins 5 and 6 limits

the LED current.

Many of the other oscillator circuits work in a similar fashion.

If voltage boost is not needed (with or without current limit-

ing) loads can be hooked between pins 2 and 6 or pins 2

and 5.

APPLICATIONS: FLASHER & INDICATOR

Differing uses and supply voltages will require adjustment of

flashing rates. Often it is convenient to leave the capacitor

the same value to minimize its size, or to fix the pulse ener-

gy to the LED. First, the internal RC resistors can be used to

obtain 3k, 6k, or 9k by hooking to or shorting the appropriate

pins. Further adjustment methods are shown in the two

parts of

Figure 3

below.

Figure 3a

, it can be seen that the internal RC resistors are

shunted by an external 1k between pins 8 and 4. This will

give a little over 3 times the flashing rate of the typical 1.5V

flasher of

Figure 1

The 3.9k resistor in

Figure 3b

connected from pin 1 to the

6V supply raises voltage at the bottom of the 6k RC resistor.

Charging current through that resistor is greatly reduced,

bringing flashing rate down to about that of the 1.5V circuit

(1 Hz). As will be explained later, this biasing method also

insures starting of oscillation even under unfavorable condi-

tions.

TL/C/7213–5

FIGURE 3a. Fast Blinker

TL/C/7213–6

FIGURE 3b. 6 Volt Flasher

Two precautions are taken for circuit reliability. The added

75Xseries resistor for the LED keeps current peaks within

safe limits for the diode and IC. Also, in operation above a

3V supply, the electrolytic capacitor sees momentary volt-

age reversals. It should be rated for periodic reversals of

1.5V.

A continuously appearing indicator light can also be pow-

ered from a single 1.5V cell as shown in

Figure 4.

Duty cycle

and frequency of the current pulses to the LED are in-

creased until the average energy supplied provides suffi-

cient light. At frequencies above 2 kHz, even the fastest

movement of the light source or the observer’s head will not

produce significant flicker.

Since this indicator powering circuit uses the smallest ca-

pacitor that will reliably provide full output voltage, its oper-

ating frequency is well above the 2 kHz point. The indicator

is not, however, intended as a long life system, since battery

drain is about 12 mA.

High frequency operation requires addition of

two

external

resistors, typically of the same value. One, of course, shunts

the high internal timing resistors. If only this one were used,

the capacitor charging current would have to pass through

the two 400Xresistors internally connected between pin 5

and the collector of Q3. Oscillation at a slower rate and

lower duty cycle than desired would occur, and oscillation

might cease altogether before the battery was fully dis-

charged. The second 68Xresistor shunting the two 400X

resistors eliminates these problems.

The circuit in

Figure 5

is a relaxation type oscillator flashing

2 LEDs sequentially. With a 12 VDC supply, repetition rate is

2.5 Hz. C2, the timing and storage capacitor, alternately

charges through the upper LED and is discharged through

the other by the IC’s power transistor, Q3.

If a red/green flasher is desired, the green LED should have

its anode or plus lead toward pin 5 (like the lower LED). A

shorter but higher voltage pulse is available in this position.

TL/C/7213–7

FIGURE 4. ‘‘Continuous’’ 1.5V Indicator

TL/C/7213–8

FIGURE 5. Alternating Flasher

Indication or monitoring of a high voltage power supply at a

remote location can be done much more safely than with

neon lamps. If the dropping resistor (43k as in

Figure 6

)is

located at the source end, all other voltages on the line, the

IC, and the LED will be limited to less than 7V, above

ground.

The timing capacitor is charged through the dropping resis-

tor and the two 400Xcollector loads between pins 2 and 5

of the IC. When capacitor voltage reaches about 5V, there

is enough voltage across the 1k resistor (to pin 8) to turn on

Q1, and hence trigger on the whole IC to discharge the ca-

pacitor through the LED.

TL/C/7213–9

FIGURE 6. Safe, High Voltage Flasher

TL/C/7213–10

FIGURE 7. ‘‘Mini-Strobe’’ Variable Flasher

There are many other LED applications and variations of

circuits. A chart outlining operation of the circuit of

Figure 6

at various voltages appears on the LM3909 data sheet. Also

shown are circuits for adjusting the flash rate, flashing 4

LEDs in parallel, and details for building a blinking locator

light into an ordinary flashlight.

Incandescent bulbs can also be flashed, as already illustrat-

ed in

Figure 1

. However, most such bulbs draw more than

the 150 mA that the LM3909 can switch. The two following

circuits therefore use an added power transistor rated at 1A

or more. In each circuit, an NPN transistor is used, so the

power transistor’s base drive is obtained from the common

or ground pin of the flasher IC.

The 3V ‘‘mini-strobe’’ of

Figure 7

may be used as a variable

rate warning light or for advertising or special effects. The

rate control is so wide range that it adjusts from no flashes

at all to continuously on. Chosen for rapid response, the

miniature 1767 lamp can be flashed several times a second.

A ‘‘mini-strobe’’ circuit was tested in a Lantern Flashlight

with a large reflector. In a dark room, the flashes were al-

most fast enough to stop a person’s motion. As a toy, the

fast setting can mimic the strobes at rock concerts or the

flicker of old-time movies.

Figure 8

below shows a higher power application such as

would use an automotive storage battery for power. It pro-

vides abouta1Hzflash rate and powers a lamp drawing a

nominal 600 mA.

A particular advantage of this circuit is that it has only 2

external wires and thus may be hooked up in either of the

two ways shown below in

Figure 9

. Further, no circuit failure

can cause a battery drain greater than that of the bulb itself,

continuously lit.

In the circuit of

Figure 8

, the 3300 mF capacitor performs a

number of other functions. It makes the LM3909 immune to

supply spikes, and provides the means of limiting the IC’s

supply voltage. Since the LM3909 can only operate with

TL/C/7213–11

FIGURE 8. 12 Volt Flasher (2 Wire)

Note: If flasher case insulated, it will TL/C/7213–12

operate in positive or negative

ground systems.

FIGURE 9. 2 Wire Flasher Usage

7.5V or less on pin 5 (in this circuit) the 200X/1.3k divider

attached to pin 8 of the IC causes it to turn fully on at 7V or

less on pin 5. Then the LM3909 discharges the timing ca-

pacitor (its own supply voltage) to 4V or less, whereupon it

turns off. The capacitor discharge current comes out of pin

4 of the IC, turning on the NSD U01 transistor. It is the large

size of the timing capacitor that allows it to

store

all the

needed energy for turning on the power transistor. This in

turn permits the whole flasher circuit to operate as a 2 wire

device.

Many other flasher possibilities exist. LED flash rate can be

varied from 0 to 20 Hz, or a number of LEDs may be flashed

in parallel. With a 3V supply, yellow and green LEDs may be

flashed. A 6V incandescent ‘‘emergency lantern’’ can be

made and its PR-13 bulb may be made to give continuous

light or flash by switch selection. This is a more reliable,

longer lived system than a lantern with a second thermal

flasher bulb. The NSL4944 Current Regulated LED makes

possible flashing many LEDs in parallel or with high volt-

ages without series resistors.

APPLICATIONS: Audio & Oscillator

Very economical continuity checkers, tone generators, and

alarms may be made from the LM3909. No matching trans-

former is needed because the 150 mA capability of the

LM3909 output can drive many standard permanent magnet

(transistor radio) loudspeakers directly. The 1.5V battery

used in most applications is both lower in cost and longer

lasting than the conventional 9V battery.

In the continuity checker of

Figure 10

, a short, up to about

100X, across the test probes provides enough power for

audible oscillation. By probing 2 values in quick succession,

small differences such as between a short and 5Xcan be

detected by differences in tone.

A novel use of this circuit is found in setting the timing of

certain types of motorcyles. This is due to the difference in

tone that can be heard from the tester depending whether

there is a short or not across the low resistance primary of

the ’cycle’s ignition coil. In other words, the difference be-

tween a 1Xresistor and a 1Xinductor can be heard. Quick

checks for shorts and opens in transformers and motors

can therefore be made.

Darkrooms, laundry rooms, laboratories, and cellar work-

shops can often suffer damage from spills or water seepage

ruining lumber, chemicals, fertilizer, bags of dry concrete,

etc. The circuit of

Figure 11

is safe on potentially damp

floors since there is no connection to the power line. Fur-

ther, its standby battery drain of 100 mA yields a battery life

close to (or, according to some experiments, exceeding)

shelf life.

Without moisture, multivibrator transistor Qais completely

off, and its collector load (6.2k) provides enough current to

hold pin 8 of the LM3909 above 0.75V where it cannot oscil-

late. When the sense electrodes pass about 0.25 mA, due to

moisture, Qastarts turning on, and since Qbis already par-

tially biased on, positive feedback now occurs. Qaand Qb

are now an astable multivibrator which starts at about 1 Hz

and oscillates faster as more leakage passes across the

sense electrodes.

This ‘‘multi’’ then acts as both an amplifier and a modulator.

The pulse waveform at the collector of Qavaries the timing

current through the 3.9k resistor to pin 8 of the LM3909

resulting in a distinctively modulated tone output.

The sensor should be part of the base of the box the alarm

circuitry is packaged in. It consists of two electrodes six or

eight inches long spaced about (/8 inch apart. Two strips of

stainless steel on insulators, or the appropriate zig-zag path

cut in the copper cladding of a circuit board will work well.

The bare circuit board between the copper sensing areas

should be coated with warm wax so that moisture on the

floor,

not

that absorbed by the board, will be detected. The

circuit and sensor can be tested by just touching a damp

finger to the electrode gap.

Minimum cost, simplicity, and very low power drain are the

aims of the Morse Code set of

Figure 12.

One oscillator

simultaneously drives speakers at both sending and receiv-

TL/C/7213–13

FIGURE 10. ‘‘Buzz Box’’ Continuity and Coil Checker

TL/C/7213–14

FIGURE 11. Water Seepage Alarm

TL/C/7213–15

FIGURE 12. Morse Code Set

ing ends. Calculations and actual use tests indicate life of a

single alkaline penlight cell to be 3 months to over a year

depending on usage. ‘‘Buzzer’’ type sets use two or more

batteries with much shorter life.

Commonly available, low cost 8Xspeakers are effectively in

series to better match LM3909 characteristics. The three

wire system and parallel telegraph keys allow beginners and

children to use the set without having to understand use of a

‘‘send-receive’’ switch.

The two resistors are added to obtain a suitable average

power output and electrically force the oscillator toward the

desired 50% duty cycle. Acoustically, both speakers are op-

erated at resonance (about 400 Hz in the prototype) for

maximum pleasing tone with minimum power drain. Each of

the two speaker enclosures has holes added to augment

this resonance. For each different type or brand of speaker

and size of box, hole and capacitor sizes will have to be

determined by experiment for the most stable resonant tone

over the expected battery voltage variation.

Experiments with the above circuit led to development of

circuit in

Figure 13.

It is optimized to oscillate at any

acous-

tic

load frequency of resonance! With just a speaker, oscilla-

tion occurs at the speaker cone ‘‘free-air’’ resonance. If the

speaker is in an enclosure with a higher resonant frequency

. . . this becomes the frequency at which the circuit oscil-

lates.

An educational audio demonstration device, or simply an

enjoyable toy, has been fabricated as follows. A roughly cu-

bical box of about 64 in.3was made with one end able to

slide in and out like a piston. The box was stiffened with thin

layers of pressed wood, etc. Minimum volume with the pis-

ton in was about 10 in.3. Speaker, circuit, battery, and all

were mounted on the sliding end with the speaker facing out

through a 2(/4 in. hole. A tube was provided (2(/2 in. long, ±/16

in. ID) to bleed air in and out as the piston was moved while

not affecting resonant frequency.

‘‘Slide tones’’ can be generated, or a tune can be played by

properly positioning the piston part and working the push

button. Position and direction of the piston are rather intui-

tive, so it is not difficult to play a reasonable semblance of a

tune after a few tries.

The 12Xresistor in series with pin 2 (output transistor Q3’s

collector) and the speaker, decouples voltages generated

by the resonating speaker system from the low impedance

switching action of Q3. The 100 mF feedback capacitor

would normally set a low or even sub-audio oscillation fre-

quency. Therefore, the major positive feedback voltage to

pin 8 is the resonant motion

generated

voltage from the

speaker voice coil. Therefore the LM3909 will continue to

drive the speaker at the resonance with the highest com-

bined amplitude and frequency.

It can be seen already that the LM3909, having direct

speaker drive and resonance following capability, can do

things that are a lot less practical with older timer and uni-

junction circuitry. Two final ‘‘sound effect’’ type of circuits

are illustrated in

Figure 14.

TL/C/7213–16

FIGURE 13. Electronic ‘‘Trombone’’

The siren of

Figure 14a

produces a rapidly rising wail upon

pressing the button, and a slower ‘‘coasting down’’ upon

release. If it is desirable to have the tone stop sometime

after the button is released, an 18k resistor may be placed

between pins 8 and 6 of the IC. The sound is then much like

that of a motor driven siren.

In this circuit, the oscillation must not be influenced by

acoustic resonances. The 1 mF capacitor and 200Xresistor

determine a pulse to the speaker that is wider than that for

flashing LEDs, but much narrower than is used in the tuned

systems of

Figures 12

and

13.

The repetition rate of speak-

er pulses is determined by the 2.7k resistor, and the charge

on the 500 mF capacitor. Discharging this capacitor with

the pushbutton increases current in the 2.7k resistor caus-

ing a rapid upshift in tone.

The ‘‘whooper’’ of

Figure 14b

sounds somewhat like the

electronic sirens used on city police cars, ambulances, and

airport ‘‘crash wagons.’’ The rapid modulation makes the

tone seem louder for the same amount of power input.

The tone generator is the same as in the previous siren.

Instead of a pushbutton, a rapidly rising and falling modulat-

ing voltage is generated by a second LM3909 and its asso-

ciated 400 mF capacitor. The 2N1304 transistor is used as a

low voltage (germanium) diode. This transistor along with

the large feedback resistor (5.1k to pin 8) forces the ramp

generator LM3909 into an unusual mode of operation hav-

TL/C/7213–17

FIGURE 14a. Fire Siren

TL/C/7213–18

FIGURE 14b. Whooper Siren

ing longer ‘‘on’’ periods than ‘‘off’’ periods. This raises the

average tone of the tone generator and makes the modula-

tions seem more even.

APPLICATIONS: Trigger & Other

With its high pulse current capability, the LM3909 is a good

pulse-transformer driver. Further, it uses fewer parts and

operates more successfully from low voltage supplies than

do the equivalent unijunction circuits. The ‘‘Triac’’ trigger of

Figure 15

operates from a 5V logic supply and provides gate

trigger pulses of up to 200 mA.

With no gate input, or a TTL logic high input, the LM3909 is

biased off since pin 1 is tied to Va. With a logic low at the

gate in, the IC provides 10 ms pulses at abouta7kCrate. A

TTL gate loaded only by this circuit is assumed since other-

wise worst-case voltage swing may be insufficient. This trig-

ger is not of the ‘‘Synchronized Zero Crossing’’ type since

the first trigger pulse after gating on could occur at any time.

However, the repetition rate is such that after the first cycle,

a triac is triggered within 8V of zero with a resistive load and

a 115 VAC line.

The standard Sprague PC mounting transformer provides a

2:1 current step-up, and suitable isolation between the low

voltage circuitry and power lines up to 240 VAC. Resistor

Rg, which includes transformer winding resistance, can be

as little as 3 or 4Xfor high current triacs. Low current types

may need excessive ‘‘holding’’ current with such low Rg,so

it may be raised to as much as 100Xwith a sensitive gate

triac.

Oscillation of the LM3909 will start when the DC bias at pin

8 is between 1.6 and 3.9V. In

Figure 15,

pin 8 is connected

between the 10k input resistor and a 6k resistor to

5V. With 3.8V in, pin 8 is at 4.5V so there is no oscillation.

With 1V, or less, in, pin 8 is at 3.5V or below and oscillation

occurs. From this example, it can be seen that other input

resistors or bias dividers can be calculated to gate the

LM3909 triac trigger from other logic levels.

A useful electronic lab device is a precision square wave

generator/calibrator. If the output is held at a few tenths

percent of 1V, peak-to-peak, it is useful in calibrating oscillo-

scopes and adjusting ’scope probes. Many lower cost or

battery-portable oscilloscopes do not have this feature built

in. Also it is useful in checking gain and transient response

of various amplifiers including ‘‘hi-fi’’ power amplifiers.

Battery powered equipment is free from both the inconve-

nience of a line cord, and from some of the noise and hum

effects of equipment attached to the power line. Operation

for over five hundred hours from a single flashlight ‘‘D’’ cell

is the bonus provided by the circuit of

Figure 16.

The lowest

reference voltage regulator available, the LM113, is used in

conjunction with a current source, and the voltage boost

characteristic of the LM3909.

Output is a clean rectangular wave which can be adjusted to

exactly a 1V amplitude. A rectangular wave of approximate-

ly 1.5 ms ‘‘on’’ and 5.5 ms ‘‘off’’ was chosen for circuit sim-

plicity and low battery drain. Waveform clipping is virtually

flat due to complete turn-off of the current switch Q2and

the typical ‘‘on’’ impedance of 0.2Xprovided by the LM113.

The 0.01% temperature coefficient of the LM113 at room

temperature allows negligible drift of the waveform ampli-

tude under laboratory conditions. Loading by a ’scope probe

will also be insignificant.

The circuit will work properly down to battery voltages of

1.2V. This is because the 100 mF electrolytic capacitor

drives the emitter of Q2below the supply minus terminal. At

a battery voltage of 1.2V, the collector of Q2can still swing

more than 1.6V. Q1uses the ‘‘off’’ periods of the LM3909 to

insure that the 100 mF capacitor is charged to almost the

TL/C/7213–19

FIGURE 15. Triac Trigger

TL/C/7213–20

FIGURE 16. ’Scope Calibrator

TL/C/7213–21

FIGURE 17. R.F. Oscillator

entire battery voltage. Thus when the LM3909 turns on and

pin 2 drives almost to the minus supply voltage, the negative

side of the capacitor is driven 0.9 to 1.2V below this termi-

nal. Low battery voltage cannot lead to an undetected error

in the 1V squarewave. This is because the waveform be-

comes distorted rather than just decreasing in amplitude as

battery voltage becomes too low.

Taking advantage of the versatility and the indestructability

of the LM3909 by a 1.5V battery, the IC can become an

ideal teaching means, or experimental device for the young

electronic hobbyist. As well as the circuits already present-

ed, the LM3909 can be made to work as amplifier, radio,

and even logic type circuits. The ideas of negative and posi-

tive feedback can be presented. The circuits presented in

Figures 17

through

are intended as illustrations or dem-

onstrations of circuitry concepts such as would be used in

an experimenter’s kit. They are not meant to be used as

parts of finished commercial products with specific perform-

ance specifications. In other words, working circuits have

been breadboarded, but no measurements of performance

such as frequency range and distortion have been attempt-

ed.

Both tuned circuits of

Figures 17

and

use standard AM

radio ferrite antenna coils (loopsticks) with a tap 40% of the

turns up from one end. The oscillator works up to 800 kHz

or so, and the radio tunes the regular AM broadcast band.

Both also use standard (360 pF) AM radio tuning capacitors.

The oscillator has the normal capacitive positive feedback

used with LM3909 circuits, but with frequency determined

by the tuned circuit loading the output circuit. Detailed oper-

ating descriptions of these experimenter’s circuits will not

be attempted in order to keep down the length of this note.

Near the end, a discussion of the IC’s general theory of

operation will be given, which should help in understanding

the individual circuits.

In the radio circuit of

Figure 18,

the LM3909 acts as a detec-

tor amplifier. It does not oscillate because there is no posi-

tive feedback path from pin 2 to pin 8. The tuning ability is

only as good as simple ‘‘crystal set,’’ but a local radio sta-

tion can provide listenable volume with an efficient 6 inch

loudspeaker. Extremely low power drain allows a month of

continuous radio operation from a single ‘‘D’’ flashlight cell.

Antennae for the radio circuit can be short (10 to 20 feet)

and connected directly to the end of the antenna coil as

illustrated. Longer antennae (30 to 100 feet) work better if

attached to the previously mentioned tap on the coil . . . also

illustrated.

The following two circuits are examples of logic or computer

type functions. They use 3V power supplies (2 cells) be-

cause the LM3909 was designed not to have any stable or

‘‘latching’’ states with a 1.5V supply.

Switches on both the above circuits are momentary types.

In each case a small charge or impulse affects the circuit’s

state. The circuit of

Figure 19

switches to and

holds

its con-

dition whenever the switch changes sides, even if contact is

made only very briefly. The circuit of

Figure 20

delivers

about a (/2 second flash from the LED every time its push-

button makes contact, whether briefly or for a much longer

period of time. Such circuits are used with keyboards, limit

switches, and other mechanical contacts that must feed

data into electronic digital systems.

By again leaving out the positive feedback capacitor, the

LM3909 can become a low power amplifier. This little audio

amplifier can be used as a one-way intercom or for ‘‘listen-

ing in’’ on various situations. Operating current is only 12 to

15 mA. It can hear fairly faint sounds, and someone speak-

ing directly into the microphone generates a full 1.4V peak-

to-peak at the loudspeaker.

TL/C/7213–22

FIGURE 18. Radio

TL/C/7213–23

FIGURE 19. Latch Circuit

TL/C/7213–24

FIGURE 20. Indicating One-Shot

TL/C/7213–25

FIGURE 21. Mini-Power Amplifier

APPLICATION HINTS

With 1.5V supplies, certain problems can occur to stop os-

cillation or flashing. Due to the way gain is achieved and the

type of feedback, too heavy a load may stop an LM3909

from oscillating. 20Xof pure resistive

load

will sometimes

do it. Strangely enough, lamp filaments, probably because

of some inductance, don’t seem to follow this rule. Also in

flasher circuits, an LED with

leakage

or conductivity be-

tween 0.9 and 1.2V will stop the LM3909. Maybe 1% of

LEDs will have this defect because they are not often tested

for it.

Greater frequency stability was not one of the design aims

of the LM3909. In LED flasher circuits it is better than might

be expected because the negative temperature coefficient

of the LED partially compensates the IC. We planned it this

way. Simple oscillators, without the LED, are uncompensat-

ed for temperature. This is due to using 1 )/3 of a silicon

junction drop as the on-off trip point and the use of the

integrated timing resistors with their positive temperature

coefficient. Further, most capacitors of 1 mF or over, shown

in the circuits, will usually be electrolytics for size reasons.

These, however, are not particularly stable with temperature

and their initial tolerances vary greatly with type of capaci-

tor.

In most of the oscillator circuits, frequency is also propor-

tional to battery voltage. This must be considered when

starting with a completely unused cell at 1.54V or so and

deciding what the ‘‘end-of-life’’ voltage is to be. This can be

in the range of 1.1 to 0.9V, a drastic change. It helps to

remember how bright flashlights are with a fresh set of bat-

teries, and how dim they are when the batteries are finally

changed.

Flashers and tone generators for alarms are not, however,

demanding for stability. Flash rate changes of 50% or tone

shifts of (/2 an octave are not particularly annoying or even

too noticeable.

One interesting point is that the low operating power of

most of the circuits presented allows them to be powered by

solar cells

as well as regular batteries. In bright sunlight, 3 to

4 cells in series will be needed. In dimmer light, 4 to 6 cells

will do the job. Current from cells way under an inch in area

generally will be sufficient, but circuits drawing a high pulse

current (such as SCR triggers) will need a surge storage

capacitor across the solar cell array.

The LM3909 was designed to be inherently self-starting as

an oscillator, and LED flasher circuits

are

, at any voltage,

because the load is nonlinear. A load with sufficient self

inductance will always self-start, although possibly at a high-

er than expected frequency. There is an exception for large-

ly resistive loads on an oscillator operating with a supply

larger than 2 or 2.5V. A stable state exists with Q3turned

completely ‘‘on’’ and the timing resistors from pin 8 to the

supply minus still drawing current. A reliable solution is to

bias pin 8 (for instance with a resistor to Va) so that its DC

voltage is one half V less than half the supply voltage.

The duty cycle of the basic LED flasher is inherently low

since the timing capacitor is also driving the very low LED

‘‘on’’ impedance. For other oscillators the ‘‘on’’ duty cycle

can be stretched by adding resistance in series with the

timing capacitor. Additionally, nonlinear resistance can be

used as timing resistance. (See

Figure 14b

)

CONCLUSION

Applications covered in this note range in use from toys to

the laboratory, and in frequency from DC to RF. The

LM3909 can be used to amuse, teach, or even upon occa-

sion to save a life. As a practical cost consideration the

LM3909 IC can often replace a circuit having a number of

transistors, associated parts, and high assembly cost.

Further, the LM3909 demonstrates the practicality of very

low voltage electronic circuits. They can work at high effi-

ciencies if ingenuity is used to design around transistor junc-

tion drops. In such circuits stresses on parts are so low that

extremely long life can be predicted. Often transistors, ca-

pacitors, etc. that would be rejects at higher voltages can be

used. Voltage dividers, protective diodes, etc. often needed

at higher voltages can be left out of designs. Power drains

are so low that circuits can be made that will last months to

years on a single cell.

A single cell is more reliable and has a higher energy densi-

ty then multiple cells. This is due to lack of cell interconnec-

tions and insulation as well as elimination of packaging to

hold multiple cells in place.

AN-154 1.3V IC Flasher, Oscillator, Trigger or Alarm

LIFE SUPPORT POLICY

NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT

DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF NATIONAL

SEMICONDUCTOR CORPORATION. As used herein:

1. Life support devices or systems are devices or 2. A critical component is any component of a life

systems which, (a) are intended for surgical implant support device or system whose failure to perform can

into the body, or (b) support or sustain life, and whose be reasonably expected to cause the failure of the life

failure to perform, when properly used in accordance support device or system, or to affect its safety or

with instructions for use provided in the labeling, can effectiveness.

be reasonably expected to result in a significant injury

to the user.

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