two transistor flasher circuit - reverse engineering

[armandine2]

I have recently drawn out a simple printed circuit board, to understand an old Uniross Ni-Cd rechargeable torch.

I have recreated the flasher circuit onto a breadboard and connected it to the torch head - where it flashes about once a second, with the pcb disconnected.

However, trying this breadboard circuit with apparently similar bulbs I have it just lights the bulb without flashing.

At first I thought I wonder if the torch head - which is glued together and likely difficult to disassemble - has another component inside. Or is the circuit quite well designed to the bulb's resistance?

I have as yet [not] figured out the circuit's operation - any ideas?

[edited]

[ledtester]

Or is the circuit quite well designed to the bulb's resistance?

Here is a Falstad simulation:

(Falstad simulation)

The bulb is represented by the 10R resistor. To speed things up I used a 1uF cap.

The circuit seems to be sensitive to the bulb's resistance. At 20R it doesn't oscillate anymore.

[armandine2]

The original bulb was measured at 1.0 ohm - I don't think the other bulb I measured was a great deal more, but enough I guess. 

[tooki]

The original bulb was measured at 1.0 ohm - I don't think the other bulb I measured was a great deal more, but enough I guess. 

The cold resistance of a bulb is much lower than once hot. In Falstad, you can actually place an incandescent lamp (not sure why ledtester didn’t just do that, actually), and you can see its resistance change as it heats up. If I set it to a 3V, 1W lamp, then it starts with a cold resistance of 0.45 ohms and settles in at 9 ohms, just like Ohm’s law would predict.

[Konkedout]

That circuit looks like it can probably work.  But I wonder about one or two diodes in the pcb layout which I do not see in the schematic?

If you had >4V or so you could use a white LED.  That would probably require a low value resistor in series and maybe also a high value resistor in parallel.

[armandine2]

The two wires on the left hand side go to the charger socket - the diodes isolate the charger circuit from the light on and flasher circuits.