banging my head, 1 wire, 2 functions

dualsport wrote: You can put your two 470 ohm resistors in parallel to maintain the same current to make sure you drive the transistors into saturation. If your relays require a higher current you should decrease the resistor value to get more drive current. Measure the Vce voltage across the transistor that's turned on to make sure it's driven fully on, should be close to zero volts.

You should be able to leave the resistor right at 470 as long as it is a 12vdc system.  These transistors (from memory) have a gain of 35 so saturation really isn't an issue (theoretical output at 12vdc and 470 ohms is ~900mA).

Can you explain how the top PNP transistor will be biased through the zener?  I understand that the bottom zener will start conducting after the break down voltage is passed (6v in this case) but how is the top zener going to start conducting the (-) signal required to bias the PNP 3906?  Also, if you have 12vdc feeding from the collector of the 3906 to the base won't it avalanch both diodes anyway?  I'll have to add this to my list of circuits to build to see if it works out - I must say I'm skeptical at this point.

It's the same thing with the top zener when the input being supplied is a ground signal.  As long as the voltage across the zener is greater than the breakdown voltage, it'll start to conduct and pass current through.

For the top zener, the voltage being applied is 12V in the standby condition, and then when the ground signal is applied, the anode of the diode tries to go to zero through the biasing resistor, so it'll conduct and maintain the 6.2V across it.   Pretty much like ground switching a relay with the other side of the coil at 12V vs. switching it on by applying 12V to it with the other side of the coil grounded.

Because the zener drops the voltage by about 6V, then you would need to use 6V as the driving source for calculating the bias resistor requirement.  If the gain is 35, and the relay needs 200mA, then 6V would be ample for the base current.

Since you already built up the original circuit it should be easy enough to try out.  Give it a whirl and see- mouser has the diodes for pennies- :)

If you have a very high supply voltage, you may want to use a higher zener voltage to prevent conduction in the standby condtion (basically the two zener voltages added up should be greater than the supply voltage)