The Cooling Fan
A useful addition to any reflector telescope is a fan to cool down the mirror quickly. This is how I did it.
I had a small fan from an old computer CPU so all I needed was a power supply and a rheostat to control the fan speed. Well that was the theory until I learned about PWM and how to use it to control a fan's speed.
On the back of the 200p there are screw holes to fit a back-plate. I cut one from a piece of 3 mm birch plywood and cut a hole in the middle to mount the fan.
This is the finished Job.
All that was needed was the PWM circuit. This is built around the 555 timer chip which is set up to send pulses to the fan motor which causes it to rotate. The frequency of the pulses is controlled by a variable resistor. The advantage of this circuit is that the fan can rotate fairly slowly without stalling as the pulses are at almost the supply voltage. Ideal for when the mirror has cooled down but some airflow is still needed. A second advantage is there is no energy wasted through excess heat as would happen with a rheostat.
This is the controller circuit I used for the fan controller. It is based on a 555 timer chip running in astable mode. Timing is done via the 1k feed back resistor and the 50k pot. I won't attempt a more technical explanation here, suffice to say there are many such circuits to be found on the WWW which have detailed explanations on how they work.
If you use strip-board as I did plan the layout carefully and it goes without saying solder carefully to avoid bridging