Power & Control

Of wiper motors, power supplies,and relays.

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Windshield Wiper Motor 

Let's take a moment and reflect on the unappreciated windshield wiper motor. Great torque, dual windings, quiet operation for under $20 - this baby is made for Halloween. The wiper motor as removed from the car has a 'park' position that gives you a clunk every revolution. It is simple to open the case of the gear assembly and take out the cam that caused the thump. I mounted mine to a 3/4" plywood base that has proven handy for installing the motor to a variety of props. It also helps electrically isolate the motor case (DC ground) from any other AC prop accessories, such as the PC power supply below.

This is the schematic for the entire wiper/washer circuit in a GM vehicle. The wiper/washer switch is not part of the wiper motor assembly but is included because it identifies the function of timer and cycle switches built into the motor assembly. The only connections that I have used for props are the Purple (+12 volt for high speed), Yellow (+12 volt for low speed) and Black (12 volt chassis ground). These wires all come from a three-pin connector on the motor itself. Note that you would connect either the Purple wire or the Yellow wire to +12 volt, depending on your desired speed, but never both. 


Scary Terry - Wiper Motor

Howstuffworks - Wipers


PC Power Supply 

The great companion to the wiper motor - an old PC power supply. This one provides three output voltages rated at 10 amp each - 12v (yellow wires), 5v (red wires), and 3.3v (orange wires). The great thing is that the three output voltages and the wiper motor 's dual windings in combination produce quite a range of RPMs. For instance, the 5 volt output connected to the low RPM motor wire produced the perfect speed for the Flying Crank Ghost.

I mounted a nylon terminal strip to the case of the power supply and terminated the output wires there to make it easier to use in a variety of props.



Here is a simple cycle control circuit for a two-stage prop. Requires two 12 volt relays and two momentary switches (with "normally closed" terminals).

Switch 1 is mounted to be actuated when the prop is in the "at rest" position. Switch 2 is mounted to be actuated when the prop arrives at the "full scare" position. A negative trigger (either from you or a motion sensor, etc.) causes Relay 1 to 'latch', and in turn activates the Relay 2 to provide 12 volt power to the wiper motor. When the prop pops up or does whatever its thing is and hits Switch 2, the latched relay deactivates and stops the motor. The prop is then reset by a second negative trigger (either from you or a timer) which provides power to the motor to return the prop to its ready position.

If your prop requires just one cycle - meaning it triggers, does its thing and returns to its original position in one continuous motion, only one momentary switch would be needed.

This configuration worked quite well and was cheap to make. NOTE: the 12-volt power feed needs to be fused for safety. 


This control circuit was posted on the HauntForum by TommaHawk. It offers all of the functionality of the first circuit and adds the ability to reverse the polarity (and therefore direction) of the motor.









 The diagram left was a hack of a cheap RC toy car control to operate a two-wheel chassis. Each wheel was powered by its own motor. There was no speed control, so it was full power forward or reverse. It provides tank-style steering.

This would not have worked well for anything requiring fine control.  It was a little clumsy using a remote with sticks laid out perpendicular to each other as a tank control, but the brain caught on eventually.









  You Gotta Love Relays! 


A relay is an electromechanical device that uses an electromagnetic coil (electro) to move switch contacts (mechanical). The coil can be energized with a small amount of current, while the switch contacts can be used for a number of applications, including switching, isolating, or polarity reversal of high current circuits.

Power transistors have gained popularity for high current switching, but lack the flexibility of  the “make and break” functions of the relay.

A typical 12-volt relay requires a coil current of .150 Amps to energize. The relay contacts can switch currents up to 30 Amps, creating a power gain as high as 200 to 1.

The classic automotive relay is the Bosch E5000. There are many variations of this design, but thankfully most manufacturers follow a standard nomenclature.


The Coil
Terminals 85 and 86 form the coil contacts. As current is passed through the coil, a magnetic field is produced, which actuates the switch contacts, causing terminals 30 and 87 to close. In order to pass current through the coil, 12 Volts must be applied to one side of the coil while Ground is present on the other. Most relay manufacturers recommend that terminal 85 should be connected to Ground, and 86 should receive 12V, but in fact the coil is non-polarized and either connection works. The only time polarity must be observed is when a “quenching diode” has been installed across the coil terminals.

The Contacts
Terminals 30, 87A and 87 form the contacts. When the relay is at rest, terminals 30 (Common) and 87A (Normally Closed) are connected. After the relay coil is energized, terminal 30 breaks its connection to 87A, and makes a connection to terminal 87 (Normally Open). Knowing this action of the contacts allows you to apply the relay in various configurations to achieve the desired results.


 Common Types of Automotive Relays are identified by poles (the number of isolated common contacts) and throws (the number of isolated connections that the wiper arm contacts). The classic relay is a single-pole (one common contact) double-throw (one normally-open and one normally-closed contact), or SPDT.



Positive Trigger to a Higher Current Positive Output




Positive Trigger to a Negative Output





 Converting a Negative Trigger to a Positive Output





Using Relays To Reverse Polarity




  Check out our hack of the Radio Shack 20-second audio recorder here!

And the lost DTMF project here!