Repeater Fan Controller

  by Mick Lindley, KB4UPI

    Below is a schematic for the fan controller used on our club's WB4YRJ VHF repeater. Since the repeater is located at my QTH and in my ham shack ("man cave", "happy spot") I did not want the fan to run all the time due to the noise. A bit of internet searching found two circuits that looked good but had multiple IC's and parts. This controller has only 1 IC and only a few parts. It is based on the tried and true IC555 timer. It is easily built on a small piece of perf board. A parts list is located below the schematic and a explanation of some of the parts and their operation. Your supply voltage will likely be 13.8 volts. This will give you close to 9 volts at the relay coil. The trigger line will work with voltage from 3 to 15 volts + or - as well as ground. This is where your PTT line will go. You can use ground for testing if you like. Increasing the resistance on R1 will increase the on time. Changing C1 to a greater value will increase the on time of the relay. The 100 mfd capacitor at C1 will give you a relay energized time of approximately 4 minutes. You can build this fan control for about $12.00. As with any building project use a socket for your 555 IC and double check all your connections before inserting your IC and powering it up. Now it's time for my standard disclaimer on all my building projects. If you build the circuit as noted it will work and work well. Don't come crying to me for any negative results. At the same time feel free to email me with questions, Mick at @ sign removed for spammers. Mick Lindley, KB4UPI

IC 555 Timer  8 Pin chip is tried and true and has been around for 35 years.
R1 One megohm pot controls the time is a high value to give you a greater range of time.
R2 10 k ohm 1/2 watt
R3 100 ohm. This resistor drops the voltage to about 9.3 volts. The diode will drop it another .3 volts.
C1 100 mfd. This sets the maximum time. Increase the value for more time. 4 min with 100 mfd.
C2 100 mfd This dampens the returning spikes from the fan when is cuts off and coasts to a stop.
C3 .01 mfd.
D1 & D2 diodes 1N914 arrests the spikes from the coil when it deenergizes.
Relay 9 volt single pole double throw. Current capability on limited by the relay contacts.