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Hey I have a circuit like this, alt text

I am powering this circuit with a 6v alkaline battery, the circuit works just fine with 100uf capacitors. But the problem is I want to replace the LED with a dpdt relay, I tried a 6v one I had laying around but it wont budge. Also when I tried the circuit with a 12v battery, the circuit just remains active for all the time. How do I get this to work?

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This should work. You may have to change the values of the resistors, as adding another transistor affects the functioning of the circuit.

Astable multivibrator relay

I have actually used a similar circuit to operate a relay at about twice a second, at 12 volts. I plan on using it for a trailer wiring tester.

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    \$\begingroup\$ So, how many transistors have you already killed this way? \$\endgroup\$ – starblue Mar 3 '10 at 20:48
  • \$\begingroup\$ @starblue I haven't had to replace a single transistor. I used a relatively small relay, so there isn't a lot of inductive feedback. \$\endgroup\$ – Brad Gilbert Mar 3 '10 at 23:21
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    \$\begingroup\$ I see that a diode appeared in the right place. :-) And, you normally don't need the resistor in series with the relay if the relay coil is rated for the supply voltage. \$\endgroup\$ – starblue Mar 4 '10 at 6:21
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You'll need to use a transistor or other switching element to drive the relay. One common circuit is here:

http://www.dnatechindia.com/index.php/Tutorials/8051-Tutorial/Relay-Interfacing.html

Just treat the output of your oscillator as the output of the microcontroller in the above circuit.

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Add a transistor to supply current to the load:

http://en.wikipedia.org/wiki/Emitter_follower

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  • \$\begingroup\$ An emitter follower? With an active GND referenced multi-vibrator a common emitter NPN circuit would be far more appropriate to this relay buffer. \$\endgroup\$ – Michael Karas Sep 18 '12 at 13:57
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    \$\begingroup\$ No, either can be made to work but an emitter-follower amplifier is more appropriate. Multivibrator circuits are disturbed by heavy loading and emitter-follower presents a higher impedance to the driving circuit than common-emitter. \$\endgroup\$ – markrages Sep 19 '12 at 3:35
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How fast are you trying to turn the relay on and off? There is a turn-on and turn-off delay. If you are operating at frequencies above 10Hz or so I would look for a different solution. If you are at frequencies below that the transistor buffer should work.

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  • \$\begingroup\$ The resistors are fairly high value (last band is orange, if I'm not mistaken...), the capacitors are largish (ie, at least 1uF, more likely 4.7uF or higher), and he's using an LED and seems to indicate that it blinks (ie, not just dim 50% PWM) so I'm guessing it's under 10Hz - and most small relays can handle 10Hz anyway. \$\endgroup\$ – Adam Davis Mar 2 '10 at 22:04
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    \$\begingroup\$ Actually @Adam, it says in the question "the circuit works just fine with 100uf capacitors." \$\endgroup\$ – Brad Gilbert Mar 3 '10 at 15:44

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