Design Philosophy

Primary batteries such as the PP3 work best if run at a fairly low, steady current, whereas to get a bright flash from
LEDs you need a current pulse of 10mA or more. Using a capacitor for energy storage solves this problem, as you can
charge it slowly through a resistor and discharge it rapidly. With careful choice of components and layout you can keep
all high current pulses out of the supply. This prevents sagging of the battery voltage when its internal resistance
increases with age.


Circuit Description

The charge path for the circuit is b.e. of T1, C2, R4. The discharge path is D1, C2, c.e. of T3.

While C2 is charging it very conveniently holds T1 turned on. However once the current falls to a level where R2 starves
T1 base, it turns off and C1 then charges via R1 and b.e. of T2/T3 turning these transistors on. This is the exact
inverse of a conventional 2 transistor multivibrator.

C2 now discharges rapidly through D1 giving a bright flash of light, and once C1 has charged fully, the process repeats.

This design of oscillator is not often used, as it is temperature sensitive and inaccurate, but it is ideal for our
purpose due to the very low biasing demands and unimportance of the precise flash rate. 


Component Choices.

T1-T3 could be any small signal silicon type with a reasonable gain. T2/T3 could in fact be a single darlington
transistor.

C1 is chosen for a compromise between low current working and stability. Lower values will cause the oscillator to 'drop
out' at a higher voltage, and give too short a pulse period for C2 to discharge properly. Higher values will broaden the
pulse width too much, increasing average current for no improved brightness.

C2 is critical for leakage reasons so MUST be tantalum, preferably 10V. A conventional electrolytic can't be used.

Do not fit a supply decoupler. It's leakage would more than double the power consumption.

D1 must be an 'ultrabright' type. Standard LEDs don't seem to like the very narrow current pulse resulting from C2
discharge. I've tried putting a resistor in series with D1 to broaden the pulse width and reduce the peak current but it
seems to be better without.

Working voltage range is from 10V down to 4.5V.

Current demand (at 9V) is calculated at 190uA peak with a measured value of 35uA average. These are very difficult
measurements to make so can't be relied on 100%

Life of a standard (non alkaline) PP3 battery is in excess of 12 months of continuous working. An alkaline one seems to
last 2-3 years.

These days I use expired smoke alarm batteries - they will still run the flasher for a very long time!


W Godfrey 1988-2006