First off let me stress that working with high voltage is DANGEROUS! Only attempt this if you are very familiar with working with high voltages. Always use the one hand rule when working with the circuit, one hand in your pocket and one hand can only be used to work on the circuit!

Just in… We just received a small supply of these disposable camera flash boards, now available while the stock lasts….
http://www.madscientisthut.com/Shopping/agora.cgi?product=Radiation%20Detection&user4=Camera%20Flash%20PCB

This is a step by step guide to modifying the disposable camera flash unit that powers the Geiger tube in the last post. Geiger tube power supply hack using disposable camera flash unit

Before we get started:

Here is a top view of the unmodified circuit board

Disposable Camera Flash Unit

Here is a bottom view of the unmodified circuit board

Disposable Camera Flash Unit

Here is the schematic of the board

Short out the bulk storage capacitor before starting, to make sure it is discharged!

Step one, two and three.
Remove the flash activation (discharge switch) wires.
Remove the battery holder.
solder power wires to the circuit board.

Step four.
De-solder and remove the bulk storage capacitor.

Step five.
De-solder the trigger transformer and flash tube wires. (yes I know, I typo-ed the picture annotation )

Step six.
Remove the flash tube and trigger transformer from the circuit board this can be done by gently squeezing the reflector assembly then pushing it carefully out of the circuit board

Step seven.
Reuse the capacitor from the trigger assembly as a output filter capacitor. Remove it from the board and place it across the traces from the bulk capacitor. This can be done by simply rotating the capacitor across to the nearest hole that is connected to the other leg of where the output capacitor was. (see image)

Step eight.
Remove the on switch (charge switch) arm

Step nine.
Place a 10K potentiometer across the charge switch trace (trace high lighted in blue)

Step ten.
Connect output wires to where the bulk storage capacitor was. (Argh…another typo in the annotation )

Here is a schematic of what was just accomplished:( oops another mistake, the switch S1 that is in the schematic is no longer there, it should just be a connection from the potentiometer to the base of the transistor)

Preset the potentiometer to mid point ~5K ohms.

Connect a multimeter to the output wires. Make sure the meter is in a high voltage range, the meter I am using is set in to 1000VDC range.

Next step is to power the unit from ~3.0VDC, the power source is up to the end user. I am using two AA batteries.

Carefully adjust the potentiometer to get an output that will power your Geiger tube. The Russian surplus SBM20 runs fine at 280-500V , so I chose to set my output to ~340VDC because at this level it draws less current from the batteries.

The next post will describe the ballast resistor and audio circuit that is in this video: