is it possible to use an npn and pnp transistor in the place of spot relay? I am controlling 2 normally closed 12vdc pneumatic valves. I need one valve to open when the other valve closes. the valves only use 100ma of current. I have some spdt relays but they are rather large for what I need.
sorry if that was overcomplicated. in short is it possible to use transistors in place of a spdt relay?
Yes, sure. Only 3 transistor, if I understand your incomplete description, you connecting two valve in series, than driving middle point to V+ or GND, alternating which valve open/close.
Design 4, 6, 21 - you would need only half of the circuit (half H-bridge).
If you not good with transistor, you may find IC (L293, L298, etc) and use only 1/4 of it's internal, usually there 4 half H-bridges.
http://www.talkingelectronics.com/projects/H-Bridge/H-Bridge-1.html
Apologies for my poor description. Thank you very much that is exactly what I was looking for, jacob