Mini projects I

Prototype board chop shop

For the last couple of months I've been working on a large dollhouse for my daughter so work on electronics stuff has pretty much halted. However I did slip in a few quick mini projects. One was a test circuit for hobby servos.

On my long list of things to try is controlling servos with a microcontroller (via assembly language). Before I started that I wanted some way of testing the various servos I have. All are bargain bin buys or salvage so I didn't have a high degree of confidence in them working. I didn't want to be banging my head against the wall debugging assembly language when the issue was with the servo.

So I did a quick internet tour and found a simple servo control circuit based on the 555 chip. (http://sarconastic.tripod.com/servodriver.html) . The site had an excellent guide to the operation and troubleshooting of the circuit. Hats off to that.

I setup the circuit on my baby green breadboard first. Hooked up the first servo and success! Servo go up, servo go down. The daughter loved playing with it to the point that the pins on the 10K pot used were almost snapped off. I did have one servo fail as it only moved a few degrees and stopped. However after taking the servo apart I remembered many years ago I had modified this servo to do continuous rotation (ie not a server, just a gearhead motor really). The passage of time...

The breadboard was cramped and messy so I decided to do the project properly (and to be honest just wanted to do a little project so I feel like I'm making progress). Since the circuit was just a 555 chip and some associated components I used a small prototype board which I cut down to size to fit in the small project box I was going to use. I used molex connectors for the servo and power plugs to be attached to. I considered having the batteries inside the project box (using a 9V battery with a 7805 regulator) but decided that since this test box would get very little use having batteries slowed discharge and leak was a bit of a waste. Also the self tapping screws used in the project box would only strip out over time. So external power was the go. The only other external part was the 10K pot used to adjust the servo position.

To support the two molex connectors (a 3 pin for the servo, a 2 pin for power) I glued a scrap piece of prototype board to the underside of the lid of the project box. I used a step drill bit to drill out the hole for the 10k pot. An hour of soldering later everything was almost working well. The prototype board I had used under the molex connectors needed a bit of probing as the glue had meant the solder didn't connect to the molex pins very well. A bit more heat and solder solved that.

Again the self tapping screws of the project box were a pain. Soft, poor quality metal screw heads.