looking for a fast flashing relay

Discussion in 'Electrical' started by RedBaronX, Feb 13, 2011.

  1. RedBaronX

    RedBaronX Member

    When I look on eBay or search with Google for "fast flashing relay" and other similar searches, I only find relays for FIXING "fast flashing" lights... and searching on Radio Shack, I'm not finding anything useful there either. If this was 20 years ago, I know that if I asked someone at Radio Shack for help, they would have a quick answer... but today's (average) Radio Shack employee probably knows little about their DIY section...

    I'm essentially going to be building a brake light/tail light with a housing that I have and some LEDs (12v system). I am thinking about making the brake light a flashing light while the running light is just steady (for a more significant difference between the running light and the braking light)

    Anyone know how/where I can find what I am looking for?


  2. loquin

    loquin Active Member

    With a small 12V DPDT relay, a resistor, and a capacitor, you can 'roll your own,' with almost any flash rate you want... Use a DPDT momentary toggle switch as the turn signal switch, and you're good to go. Refer to the attached schematic:

    First - the relay should be chosen based on relay coil resistance - the higher, the better. About the greatest resistance you can easily find is in the neighborhood of 250 Ohm for a 12V relay. The greater the resistance, the smaller value of capacitance you can get away with, (and the less power the circuit uses.)

    The Resistor should be about 1/2 the coil resistance, at 2 watts power rating. (If the coil is has a lower resistance, the resistor may need to have a larger power rating... If the coil were 100 ohm, for instance, the resistor would be 47 ohm, but it would need to handle at least 4 watts...

    You'll need to experiment with the values for R and C some.

    The capacitor will be chosen to tune the flash rate. A smaller capacitor will flash faster; a larger cap will flash slower. I would start with a 1000 uF capacitor, and go from there. I wouldn't get one with a voltage rating less than 25VDC. Note that these capacitors are polarity sensitive - make sure you connect the '-' lead to ground...

    The switch is a momentary switch, meaning that when you release it, it returns back to the center (off) position. One half of the switch provides power to the relay flash circuit, the other half selects which LED set (left or right) will be lit by the flash circuit.

    When the flash circuit is energized, current flows through R1 and charges C1. As the voltage across CI increases, eventually it gets high enough energize the K1 relay. (If R1 is too large, the voltage can never get high enough across the capacitor to pull in the relay, so, if the flash circuit doesn't start flashing, you'll need to make the relay value smaller...) Normally, a 12V relay coil will energize the relay when its voltage is in the 8 to 9 volt range.

    When the relay pulls in (energizes) the capacitor is disconnected from the charge circuit, (as the relay contact Normally Closed contact opens, and the capacitor discharges through the relay coil. Once the voltage across the capacitor drops down to about 6 volts, the relay coil doesn't have enough magnetic field to stay energized, and the relay opens. This closes the Normally CLosed contact again, allowing the capacitor to start charging, and the cycle repeats, until you aren't applying power any more.

    The second set of relay contacts simply closes and opens, pulsing the current flow through the LEDs. Note that LED turn signals normally have internal resistors, limiting current flow through them. If you are building your own LED turn signals, make sure you have either an LED driver circuit, or current limiting resistors, else you will burn out the LEDs.

    This circuit doesn't care what type of bulbs are used - it will handle filament-type bulbs at the same flash rate as LEDs.

    If you do decide to go this route, please post back with the relay coil resistance, resistor size, and capacitor sizes you end up with (at what flash rate.)

    Attached Files:

  3. RedBaronX

    RedBaronX Member

    it's actually for the brake light, not turn signals, and the switch is built in to the brake handle... anyway... it's been over 25 years since I've had to read a schematic.

    the LEDs that I got have resistors in them.
  4. loquin

    loquin Active Member

    A flashing brake light circuit is essentially the same thing, just without the DPDT switch... A SPST (NO) or SPDT, brake-light switch would feed the flash circuit.

    An instant-on flash circuit would be to have the brake switch feed both the flash circuit and the LED circuit, and have the negative side of the LED go through the normally closed (NC) contact on the 'B' relay. Ref the image below.

    Brake is pressed. The capacitor starts charging, and the LED goes on. The cap reaches the pull-in voltage of the relay, it energizes, and the brake light goes out. The cap discharges until it reaches the drop-out voltage of the relay, and the relay drops out, which turns the brake light on again. The charge/discharge cycle continues for as long as the brake is applied. When the brake is released, power is no longer fed to the LED, and it goes out immediately, no matter the state of the relay.

    Attached Files: