Electronic Circuits and Projects Forum

You pretty much got it. The relay in my circuit is just a tiny mechanical relay. The opto coupler is not an SSR but a very common opto coupler. The relay I used has a 6 volt coil. Just about any common off the shelf SPDT (Single Pole Double Throw) relay will work. If you have a local Frys they should have no problem fixing you up. I took it that your motor common (-) lead is circuit ground so drew it that way. Your change should be fine. The unloaded 8 volts will be fine, I used a 22 ohm resistor in series with the relay coil which should be fine between 7 to 8 volts and even when the battery voltage drops a little below 7 volts. Pin 3 of the relay is intentionally not used. The 1N4002 across the relay coil is there to act as a flyback diode and snub the inductive kick when the relay coil field collapses. It should work as drawn with your changes just fine.

Ron

Excellent, thank you Ron, it's easy to visualize that relay as the switch to open the + leg to the motor. I'll make a non-polarity plug pig-tail off of pin 4 of the relay and the negative lead from the output of the speed controller and that way I can swap the polarity to the motor and reverse the direction.

I assumed that the relay was a SSR, like a IC pack of sorts.

It's off to component shopping now...this should be fun. Thanks for all of your help and I'll be sure to post results.

-Roy

*Update:

For the relay, here's the data sheet: http://www.electro-tech-online.com/c...012/05/az8.pdf Wouldn't the Light Duty 3 Amp work as well (AZ8-1C-6DE)? I will have to measure the amp draw of the system under load and see. The only data I have is that the system took ouT a mere 35mAH after an hour of continuous operation.

This is U1 correct? http://search.digikey.com/us/en/prod...00-5-ND/385762

OK, as to the relay. There are a few considerations. First I wanted a high coil resistance so the relay coil current would be low. There are two reasons. The most important was how much current the opto coupler driving the relay coil could handle. I had to work within the limits of what the opto coupler could drive. That was also why I went with the more sensitive relay. The 4N25 Opto Coupler has a max forward current of about 50 mA and I wanted a margin below that max. So I wanted a coil resistance above 120 Ohms. Even though the duty will be intermittent I didn't want to push my luck. Like I mentioned any SPDT relay will work but we can't let the relay coil draw too much current. The rest of the logic was based on battery longevity.

Now Fry's may know of a better opto coupler to use with a relay drawing more current. Maybe an opto with a "Darlington Output". As long as the opto used can turn on into full saturation (full on) with the about 2 volts you have to work with and handle the relay coil current.

I suggest you show the sales rep at Frys the schematic and seek some advice. Hopefully a sharp rep will see and know what they are looking at. I don't have much time to start researching optos and other relays right now.

Ron

Let me know how it goes.

Ron