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Tech Forum

June 2015

Transistor As An STDT Switch

In a recent issue of NV, Roger Secura wrote the article “How to Use a Transistor as an SPST Switch.” My question is how can an STDT switch be made using transistors, FETs, or other non-mechanical components? If it is possible, please post a circuit.

Don Czyzyk
Santa Clara, CA


It’s pretty simple... you just need another switch that’s connected to the output of the first switch. These switches are commonly called Inverters, meaning that the output is inverted from the input. This means that for a high level input, the output is a low level, and vice-versa. So, all we need to create a bi-level output from a single input is a second inverter connected to the output of the first inverter. Bipolar transistors and FETs act pretty much the same way, the difference being that bipolar transistors are current-driven, whereas FETs are voltage-driven.

If you need more theory on transistor circuits, I refer you to a series of articles by Ray Marston in Nuts&Volts titled “Bipolar Transistor Cookbook”. This is an 8-part series that N&V has graciously made available online. www.nutsvolts.com/magazine/article/bipolar_transistor_cookbook_part_1. Be sure to have plenty of reading time available, because it contains a train-load of information. I’ve attached a circuit image depicting how to connect two transistors as an SPDT switch.

David Mason
Hazel Green, AL