What it shows:
This demo allows a lecturer to play around with various DC circuits on the overhead projector.
How it works:
A removable template of 26cm × 17cm plexiglass has a set of 6mm diameter tightly wound springs of length 1cm fixed at 5cm intervals (reminiscent of those Radio Shack® n1000-in-1 electronics kits). Standard resistors and 5cm lengths of 22AWG wire clip into these springs to form a circuit, and the template is then rested on a parent board consisting of two transparent meters (figure 1). These are connected to the template circuit by banana clips. A standard setup includes a simple circuit as illustrated in figure 2, which is accompanied by a 9V battery. With more than one template several circuits can be constructed prior to class, although in-the-field circuit building is very straight forward.
figure 1. Overhead Circuit Board with circuit template in position.
Setting it up:
Two OHP projection meters (Philip Harris P78500/9) are mounted on a 26cm × 26cm sheet of 3mm thick plexiglass. All the shunt circuitry is out of view to the audience. The leads from the meter outlets on the board are banana clip These loop over and slot easily into the ends of the springs. There is a 10k pot on the voltmeter should a lecturer want fantasy world values from the '9V' battery (use one that's running at around 8.6V with the setup we have)
figure 2. Circuit idea and Battery setup
Very clear circuitry - with the square arrangement it even looks like the ones in the text books. The only major shortcoming not sorted out as yet are the banana clip connections from the meters to the template; they're a bit messy and spoil the aesthetic symmetry.