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My goal is to make a car turn signal flasher. I used a 555 chip as the timer with the output driving a TIP122 transistor at BASE and a 100 ohm resistor between the BASE and the 555 output pin 3. The Emitter is connected to ground. The Collector is connected to one side of the the relay coil (90 ohms) and the other side of the coil to ground. Simply will not cycle. Tried different base resistors 100 to 2000 ohm. Transistor NOT switching.

BTW the flasher circuit works fine when driving an LED only. (no transistor, relay etc).

Any ideas?

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    \$\begingroup\$ Did you mean "the other side of the coil to +12V" rather than to ground? Also, no mention of the protection diode essential to avoid blowing up the transistor. We need the circuit diagram of what you have built... \$\endgroup\$ – Brian Drummond Feb 27 '13 at 16:18
  • \$\begingroup\$ can you draw a schematic, I suggest circuitlab.com \$\endgroup\$ – Kortuk Feb 27 '13 at 17:27
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You really should post a schematic instead of all the hand waving. However, there is one obvious flaw in your description. One side of the relay could should be connected to the collector of the transistor as you say, but the other side must be to the power supply, not ground.

When the transistor is off, the collector is effectively not connected or left floating. When the transistor is turned on, it will pull the collector low. Since that end of the coil will be low, the other end must be high for there to be voltage accross the coil so that the relay can turn on.

Here is how you should hook things up:

Don't leave off D1. It is there to catch the inductive kickback from the coil when it is suddenly shut off. Without it, the coil will generate high voltage and fry the transistor.

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  • \$\begingroup\$ Thank you Olin. I have D1 installed. I didn't know to attach the coil to +12 instead of ground. My circuit looks just like yours save coil to +12. Thanks again. \$\endgroup\$ – james boggs Feb 28 '13 at 1:08
  • \$\begingroup\$ Aha. I bet you had the LED also connected between collector and ground. But you see, that would forward-bias the base-collector junction (think: NPN), and so you're basically driving the LED with your input current through the transistor. That current might light the LED, but it won't be enough to activate the relay (which is why you're using a transistor). \$\endgroup\$ – Kaz Feb 28 '13 at 3:27

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