Month: April 2018

Updated motor circuit

I have updated the motor circuit with the changes I’ve made. Notably:

  • Introduced a 5v linear regulator (LM78L05A) to provide supply voltage to the S-R latch, the inverter, the Hall-effect sensors, and the Vcc for the level shifter.
  • Replaced the CD4010B with a CD4504B and connected its select pin (pin 13) to ground.

These were rookie mistakes. The latch and inverter chips need 5v in, not the 15-20v I plan to supply, and the buffer/level-shifter was entirely the wrong kind of chip (it was a high-to-low instead of a low-to-high; we want to go from the 5v logic level to the 15-20v level). Live and learn, I guess. The new schematic is pictured below.

Mounted motor video

I made some interesting discoveries in the days leading up to this video. First, I completely forgot not all of the chips in the circuit can handle more than 5 volts. I actually burned out a chip as a result (it held on bravely for a long time), so I ordered some 5v linear regulators to provide a constant 5v to these devices. I’ll update the EasyEDA schematic with them at some point.

Second, the cycle time of the logic circuit was dramatically reduced by a diagnostic LED I had connected to the output from the S-R latch. Removing this LED significantly increased the speed at which the motor turned.

Third, one of the coils is evidently shorting at voltages higher than 10. Above 10v, the current through the coils increases sharply and the motor stops spinning robustly. At first I thought this was a problem with the control circuit, but all of that stuff appears to be working correctly. Instead, I think on of the coils suddenly begins shorting when enough voltage is present. I have it narrowed down to two of them, so I just need to test them individually and see if this really is the problem. If it is, I’ll need to wind a new coil to replace that one.

Even so, the motor works really well at 10v. The following video demonstrates the current state of the project.

Crossbow mechanism

At some point, while waiting for parts to arrive in the mail, I became interested in building a small crossbow. I don’t know exactly how I’ll do the prod–the bow part–but I did succeed in making a trigger mechanism out of 1/4″ plywood tonight. It’s a rolling-nut style trigger.

I mounted it very temporarily to my drill-press block, hence all the extra holes. But the trigger does work: if you apply pressure to the notch extending up above the block, the circular nut engages the trigger. Pull the trigger and the nut slips free, releasing the shot. For a higher-poundage crossbow, you would need more leverage in your trigger. However, I plan on this one staying under 15 pounds.

If I can figure out a prod, I’ll build the crossbow and shoot some pencils with it.

Motor mounting

I mounted the motor to an official pinewood derby block over the weekend. It actually worked pretty well with the Erector wheel/axle/transmission setup I put together.

Kind of a busy photo, but you can see the gearing and the wheels. This is the rear end of the car.

I’m currently awaiting the arrival of some voltage regulator chips from Mouser. I had been getting lucky by overrating the SR latch and inverter chips with voltage, but they finally gave out, so I’m going to incorporate the regulator to hold them at the 5 volts they need. When that part arrives, I should be able to do some real testing.

Motor upgrade

Hey, I’ve been working on the motor for some time now. I got some 10mm Mn-Zn ferrite bars to use as cores and some 26 AWG magnet wire and wound new coils that each have around 3 ohms of resistance, thus eliminating the need for the big power resistors. I got it all hooked up tonight and had some really impressive torque/speed out of it.

Here are a couple of pictures. The first one shows the new motor itself, and the second shows a comparison between the old nail coils and the new ferrite ones.

I finally glued that one sensor in place.

I tested the two coils with a paperclip (highly empirical) and they didn’t seem much different, but on the actual motor, they are much different. The thing is almost violently torquey now, which is good. I’ll upload a video as soon as I have someone available to help film it.