My Avatar My Avatar Sailplane Winch Solenoid Safety Buzzer

by Miami Mike, 02/13/2005.

This project was originally described on in this thread . Revisions have been made since then and are still ongoing, so please refer to this document for the latest information.

New Shrink-Wrapped Version! See below.

	Shrink-Wrapped Version


Recently our club's winch went out of control and destroyed a glider. The operator took his foot off the pedal but the motor kept running, folding the glider's wings and slamming it into the ground. Fortunately the winch had a cutoff switch and we were finally able to stop it. It happened because our winch only had one solenoid and the contacts inside had gotten stuck together. We now have two new solenoids wired in series, and I've designed a small device to help prevent this sort of thing from happening again.

There's a simple circuit by Andrew E. Mileski which was shown many years ago on his web page at, which consists of two LEDs with series resistors. An LED is connected across each winch solenoid to indicate that its contacts are open. A dark LED indicates a stuck solenoid. It's a good idea, but I believe the addition of an audible alarm makes it even better.

So, here it is:

The device incorporates Andrew E. Mileski's LED circuit, but with the additional feature that if at any time either solenoid is closed by itself, the buzzer will sound. (It's actually more of a high-pitched tone than a buzz.) If and when this happens, you can check to see which LED is dark to find out which solenoid is stuck.

Here's all the information you should need to build one for yourself:

Parts List

(2019 Update: Most or all of these Radio Shack part numbers are no longer valid.)

The parts came from Radio Shack and cost less than $16:

PC board (Image comes from Relay (Image comes from LED 1 (Image comes from LED 2 (Image comes from R1, R2 (Image comes from Buzzer (Image comes from

(Radio Shack links updated on 5/6/2006.)

Schematic Diagram

Here's the circuit:

Another illustration, incorporating the diagram on the back of the 275-232 relay package:


To test the device on your bench with a 12 volt battery or power supply, connect the red (battery side) wire to +12 volts and the brown (motor side) wire to ground. Both LEDs should light. Then touch the orange (center) wire to the red wire or +12 volts. The red light (LED 1) should turn off, the green light (LED 2) should get brighter, and the buzzer should sound. This is the indication of a stuck battery-side solenoid.

Then move the orange (center) wire over and touch it to the brown wire or ground. This time the green light (LED 2) should turn off, the red light (LED 1) should get brighter, and the buzzer should sound again. This is the indication of a stuck motor-side solenoid.

After the device is installed on your winch, test it by using a clip-lead or jumper wire to connect the input to the output of either solenoid. This will simulate a stuck solenoid. Be sure to bridge only one solenoid at a time, and for safety, make sure both LEDs are on first.

Theory of Operation

  1. With both solenoids open:
    • Assuming 13 battery volts, R1 and R2 form a symmetrical voltage divider with 6.5 volts at the center where the relay coil is connected.
    • Since LED-resistor packages LED 1 and LED 2 form a similar voltage divider on the opposite side of the coil, and the central solenoid junction connects to nothing else, no voltage appears across the coil. The relay therefore doesn't energize and the buzzer is silent.
    • The combined resistance of R1 and R2 is 150 + 150 = 300 ohms.
    • The current flow under normal rest conditions is 13 / 300 = 43 milliamps through the resistors, plus about 6 milliamps through the two LEDs, for a total of roughly 50 milliohms.
    • The total resistor power dissipation is (13 X 13) / 300 = 0.5633 watts or 0.282 watts per resistor, suggesting that 1/4 watt resistors are not quite adequate. R1 and R2 should be rated 1/2 watt or higher.
    • LED 1 and LED 2 are both lit at less than full brilliance.
  2. With the battery-side solenoid closed and the motor-side solenoid open:
    • R1 is now connected in parallel with the coil for a combined resistance of (250 X 150) / (250 + 150) = 94 ohms.
    • This combined resistance is in series with R2 for a total resistance of 94 + 150 = 244 ohms across the battery, disregarding LED 1 and LED 2.
    • Current flow through R2 is 13 / 244 = 53 milliamps.
    • The voltage across R2 is 150 X 0.053 = 8 volts. (Power dissipation is 8 X 0.053 = 0.43 watts, still safe for a 1/2 watt resistor.)
    • The voltage across the coil is 13 - 8 = 5 volts, which causes the relay to energize and sound the buzzer.
    • No voltage appears across LED 1 so it's essentially out of the circuit and dark. 13 volts appears across LED 2, lighting it to full brilliance.
  3. With the battery-side solenoid open and the motor-side solenoid closed:
    • R2 is now in parallel with the coil, and the pair are in series with R1. This condition is similar to the previous one, except that current now flows through the coil in the opposite direction, which makes no difference. As before, the relay energizes and the buzzer sounds.
    • No voltage appears across LED 2 so it's essentially out of the circuit and dark. 13 volts appears across LED 1, lighting it to full brilliance.
  4. With both solenoids closed:
    • No voltage appears across any of the circuit components, both LEDs are dark, the buzzer is silent, and the winch motor runs.

Using Different Components

The circuit needs a 5 volt reed relay with a coil resistance of at least 150 ohms, and is for use with a 12 volt battery. R1 and R2 must be equal and should have values of about half the relay coil's resistance, but any value from 40% to 60% will work. For example, 100, 120, 130, or 150 ohm resistors could be used with the 250 ohm relay listed above. If the relay specifications list the coil current in milliamps (ma) rather than the resistance in ohms, divide 5000 by the current to find the resistance.

Resistors of 100 ohms or less should be rated 1 watt or higher. Resistors of 120 to 240 ohms should be rated 1/2 watt or higher.

Alternate Case

If you can't find the Mini Project Enclosure, part #270-288, the circuit fits nicely into a two AAA cell battery holder, part number 270-414, with the metal contacts removed. (Although not shown in the picture, the case includes a cover.)

A smaller buzzer, part number 273-074, fits nicely on the outside.

When using this case, the parts fit snugly inside and no pc board is needed.

Update, 9/22/2006 - Here's another good case from Radio Shack, part #276-299:


It's called a "Bright-Red Blinking LED Module" and costs $3.69. One of these was used to build this unit, currently in use by the Southwest Soaring Group of Punta Gorda, Florida:


Update, 11/12/2009 - It may be getting hard to find those small plastic cases at Radio Shack, so I just built some buzzers using a different method: I assembled the components on 1/2 of a small Component PC Board, Part #276-149, and then covered them with shrink-wrap just like an electronic speed control (ESC).

shrink-wrap version

These units and more were donated to the Orlando Buzzards when they updated their contest winches to dual solenoids, and have been protecting sailplanes ever since.


Last update: 8/15/2019