SRS Relay + npn transistor to switch a motor

Thread Starter


Joined Apr 26, 2010
I've been working on a little project that takes a 5V DC signal from a microcontroller and uses a npn BJT driver to turn over an SRS relay (opto-isolated triac type). On the other side of the relay is connected a snubber circuit (R= 10 ohm, C=.22uf), and the 115V AC motor is connected in series. I've been using some hard coded times to turn the motor over for a specified amount of time then it shuts off. It has been working for nearly a month, but problem is now the motor won't spin. I automatically assumed my transistor or srs relay was burnt up, I swapped those for a new ones with no luck. I thought might have been the snubber circuit, so I replaced the resistor and cap as well. I checked the output signal from the microcontroller (I'm still getting 5V). When I connect the motor directly to the wall socket, it works great. I'm at my wits end trying to figure out why this thing doesn't work anymore. Could anyone give me some ideas?

Schematic Below


Joined Apr 20, 2004
Your schematic makes no sense. You have a 9 volt battery as a voltage source. A 9 volt transistor battery is not capable of operating that circuit for more than a short period. You also have a capacitor in series with the battery, and another in the output from the 7805. Capacitors block DC.

What is the purpose of the zener in the collector circuit of the transistor? You really need a current limiting resistor between the 3904 base and the PIC.


Joined Feb 4, 2008
C1 and C2 must be connected between 9V-ground and 5V-ground respectively. Also, put a 1k resistor between the output of the micro and the transistor's base. As beenthere said, replace the zener diode with a 10k resistor.