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Feb 13, 2005, 10:21 PM
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Miami Mike's Avatar
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Sailplane Winch Solenoid Safety Buzzer

Yesterday our club's winch went out of control and destroyed a glider. The operator took his foot off the pedal but the motor kept right on running. The glider's wings folded and then the winch quickly pulled it down to the ground for a spectacular crash. Fortunately we have a cutoff switch on the battery and were finally able to stop it.

Our winch only has one solenoid, and the contacts inside had gotten stuck together. Of course we're going to replace it with two new solenoids wired in series, which is what it should have had in the first place, but the other guys in the club are taking care of that. I just finished up a project of my own to go along with it.

There's a simple LED circuit by Andrew E. Mileski shown on his web page at, which is being sold as a kit by Cal Posthuma from his web page at It consists of two LEDs with series resistors, one across each solenoid, to indicate that the corresponding solenoid contacts are open. A dark LED indicates a stuck solenoid. One of our guys wanted to use that circuit, but I thought about it for a while and realized that we probably wouldn't remember to watch those LEDs, and when the time came they probably wouldn't do us much good. In my opinion, what's needed here is an audible alarm.

So, here it is:

I just finished it and tested it with my bench power supply and it appears to be working just as I planned. 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 that happens, it's only necessary to check which LED is dark and which one is lit to know which solenoid is stuck.

The device has three wires to connect:
  • Red - to the battery positive terminal.
  • Brown - to the motor positive terminal.
  • Orange - to the jumper between the two solenoids.
The parts all came from Radio Shack and cost me less than $16. Here's the list:
  1. Case, 270-288, Mini Project Enclosure, $1.99.
  2. PC board, 276-148, $1.79. (Trimmed to fit inside case.)
  3. K1, 275-232, 5VDC Reed Relay, coil resistance 250 ohms, contact rating 1 amp at 250 volts, $2.79.
  4. LED 1, 276-270, Red LED assembly with built in resistor for 12 volts, $1.99.
  5. LED 2, 276-271, Green LED assembly with built in resistor for 12 volts, $1.99.
  6. R1, R2, 271-1109, 150 Ohm, 1/2 watt, package of 5, $0.99.
  7. Buzzer, 273-059, 12VDC Piezo Buzzer, $3.29. (There were many different buzzers available that would have worked just as well.)
Here's the circuit:

If you build one of these with a different 5 volt relay, the value of R1 and R2 should be about 60% of the relay's coil resistance. You'll have to figure the necessary power rating as well. See my website for details.

Don't use a diode across the relay coil because current can flow through it in either direction.

Testing: To test the device after installing it on the winch, disconnect the control wire from one of the two solenoids and then step on the pedal. The buzzer should sound. Then replace the control wire, remove the control wire from the other solenoid, and repeat the test.

Hopefully I'll get a chance to test it on our winch with our two new solenoids next weekend. Meanwhile, I'll make all of this information plus any updates available on my website at

Last edited by Miami Mike; Feb 14, 2005 at 01:19 AM.
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Feb 13, 2005, 10:52 PM
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DeeSmith's Avatar

That is really a great idea. An audio alarm will certainly get your attention.

I have gotten Cal's kit for myself and my brother but not installed either one yet. I'm going to use his kit in place of the two LED units you specify. It should work great.

Feb 14, 2005, 12:45 AM
Healthy suntan is an oxymoron.
Miami Mike's Avatar
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That should work fine, but with the buzzer you won't need such bright LEDs, since it won't be so important for them to attract your attention.

Last edited by Miami Mike; Feb 14, 2005 at 01:17 AM.
Feb 19, 2005, 08:18 PM
Healthy suntan is an oxymoron.
Miami Mike's Avatar
Thread OP
I installed and tested the Solenoid Safety Buzzer on our winch today, and it worked perfectly. I was worried that there might be some occasional noise from the buzzer while the pedal was being tapped, but there wasn't any, and it sounded off loud and clear whenever either solenoid was bridged with a clip lead. It looks like the project is a success.

For the latest information on the Winch Solenoid Safety Buzzer, click the link at the bottom of the page at
Latest blog entry: OpenTX: Circling the Square
Apr 17, 2005, 05:09 PM
Healthy suntan is an oxymoron.
Miami Mike's Avatar
Thread OP
The project has come a long way since I first described it here two months ago. Cal Posthuma, whom I mentioned in the beginning post, is now selling these units in both kit and assembled form. He tells me he's already delivered ten units and has orders for more.

Cal's site:

My site:

Latest blog entry: OpenTX: Circling the Square

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