Top Fuel ZZ Prototype (need help)

[Steve]

So here's the idea. The PCB can't take more than 4.5 volts, so, what if we only send it like 3 volts to only control the steering, and to send a signal to trip a relay. The relay would allow us to send as much voltage as we want directly to the motor.

In the pic, I have a 3 volt lithium watch battery which runs the steering and then I have the forward and backward wires going to the relay. In the middle there is a 6 volt cell that once the relay trips, SHOULD go directly to the motor. Problem is I'm only getting 3 volts at the motor. I don't think 3 volts is enough to completely trip the 5volt relay, so it pulses off and on, but I could be wrong. Anyhow, I think the theory is sound, I just need a small enough relay with a low enough coil voltage.

any thoughts?

[Steve]

while searching for a MOSFET relay to fit my application (3v coil voltage, 6+volt current capacity) and coming up empty, I thought about an infrared photo-transistor setup from Radiohack. It would require less than 3 volts to turn the emitter on, and the detector says it will handle 20v. Size wise it should be small enough. Off to Radiohack I go...

Photo-Transistor setup

[chilli_mp138]

Steve--

How do you plan to handle the forward/reverse issue? I could see how it might be done on an SE using the directional capabilities of the LEDs. Your idea of an IR emitter/detector is imaginative. How about a self-contained opto-isolator?

chilli

[Steve]

There will be no reverse. I've actually discovered that using a single transitor instead of the photocell will work better, so in order to do that, I'll need the reverse lead off the PCB as a common emitter.

I've also decided to build a rail chassis from scratch and make this thing a full blown dragster. The setup above would work, and fit inside most stock bodies, especially since the huge relay isn't needed, but lifting the PCB each time to change the batteries would break the steering wires off more times than I care to solder them.

I've gone with a standard ZZ PCB with it's minimal features more for weight purposes than anything else. Proportional anything really isn't needed for this application.

SO, in essence, this project is really no longer a ZZ, though the initial idea could certainly be incorporated into one.

[Steve]

ok, rail dragster bad idea...steering is just too bulky. So back the stock ZZ chassis. I put a transistor in place of the relay, and it works really well for the most part, except that it sometimes loses it's mind and goes balls out for a few seconds, even after letting off the throttle. I don't know if it's because the transistor is overloaded, or the base voltage doesn't drop low enough, or something else entirely. Any ideas?

[Steve]

ok, used a different transistor and everything works fine now. This is far from practical as I'm using hearing aid and Lithium watch batteries. Here is a really quick wiring schematic of how it works. The low voltage source (1.5v-3.0v) is used for the steering, receiver, and to send the GO signal to the transistor. The high voltage source (6v+) gets dumped directly to the motor when the transistor switches.

The plan is to cram (3) 3volt lithium 2032 watch batteries in there, and use a little 1.5v hearing aid battery for steering. The batteries won't last long, but it's long enough for a terrifying trip down the drag strip.