Nuts and Volts - February 2013

■ FIGURE 4.

outboard components.

Now, turn to Figure 4 for the remaining circuitry. At the top, you’ll find the analog sensors. Temperature chip LM34DZ is a no-brainer; just hook it up directly and away you go! The output is already scaled to Fahrenheit, so the source code required for the microcontroller is very short and sweet.

The light sensor is a cadmium sulphide photoresistor, or light dependent resistor (LDR). This is tied in series with R38 to create a voltage divider. The uncommitted potentiometer, R31, is also a voltage divider. Recalling that the PIC12F683 contains ADC circuitry, then it’s equally easy to take a light reading or peg the position of the pot.

As for the IR sensor, be sure to double-check the pinout for whatever unit you’re using. In my case, pin 2 is hot while pin 3 is ground. Notice R2 and C4 which filter the supply voltage somewhat. The output is pulled up by resistor R23 and is finally made available to socket J16.

The relay is a fairly high powered device, so it’s controlled by means of transistor Q2. Rectifier D2 snubs any reverse-EMF, protecting both the transistor and the microcontroller.

The motor and lamp are also higher powered devices. In this case, we’ll use the even beefier Darlington transistor, Q3. The male header, J4, lets you choose whether the transistor controls the motor or the lamp.

Second Step: Collect the Parts

Many of the components in this circuit are commonplace; things like capacitors, resistors, and the like. You probably already have the bulk of them in your junk box but some of the more unusual items might give you pause. I had no trouble outfitting my trainer using surplus parts. In fact, one shopping trip did the trick. I basically got everything I needed for the entire project from All Electronics. If you’d like to follow suit, I have provided stock numbers in the Parts List. Do keep in mind, though, that most of these items are available from a variety of surplus dealers. Here are a few comments on several of the parts.

The PIC trainer is powered by a regulated +5V wall wart. The one I stumbled on, however, had a weird connector so I clipped it off and replaced it with a standard 2. 5 mm barrel type. This mates with PCB-mounted receptacle J1. A green LED, D8, indicates power-on status.

The IR sensor in the trainer expects to read a signal from a Sony type remote control unit. I found just such a hand controller at All Electronics for two bucks, and it works very well. Alternatively, you could use one of those universal remotes, as long as it puts out the Sony codes.

Now about the sensor. It’s the nature of the surplus business for stock to change without warning. I designed this circuit around a Vishay TSOP2138 IR sensor I purchased earlier. However, I’m now seeing one manufactured by Sharp that is probably a decent substitute. It’s possible the pinout is different, so be sure to check the datasheet before building with it.

Just so you know, most of these inexpensive IR sensors use a standard 38 kHz carrier frequency, so even if the pinout changes the firmware will remain much the same.