I broke it while trying to repair a nearby component on a 1970s board. It has two wires at one end that look like they are going to a coil and two axial wires. One of the axial wires came out with some pieces of glass.

The only markings are "P/F" which mostly likely means Philco/Ford.

The "silkscreen" was marked with "S1" and nothing else.

The end with the two coil wires is angled at 45 degrees I presume to indicate polarity.

I also need a way of working out what value it is so I can get a modern replacement.

Thanks! enter image description here

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    \$\begingroup\$ It would help to know what the device was, e.g. a TV, preferably with make and model number. \$\endgroup\$
    – Justme
    Mar 27, 2021 at 20:00
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    \$\begingroup\$ Sounds to me like a reed relay - the relay contacts would be sealed in a glass tube inside the coil. \$\endgroup\$ Mar 27, 2021 at 20:03
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    \$\begingroup\$ Vehicle cruise control module. Make is Philco/Ford circa 1974. I don't have a model number. \$\endgroup\$
    – Andy
    Mar 27, 2021 at 20:04
  • \$\begingroup\$ Reed relay. You can replace the switch that used to live in the hole in the middle; that's what the bits of glass were. \$\endgroup\$
    – user16324
    Mar 27, 2021 at 20:30
  • \$\begingroup\$ If you haven't, you should search for a circuit diagram and if you find it, add it to your question. Automotive diagrams are easier to find than most. Older circuitboards are less likely to be complicated, digital, or have obfuscated components, so tracing the circuit and producing a circuit diagram is not unreasonable. One plausible purpose for a relay is to latch the cruise control on with a momentary contact button. Another is to break the latch circuit when the brakes are pressed. If you trace the circuit you can probably figure out what type of relay you need based on connection. \$\endgroup\$
    – K H
    Mar 28, 2021 at 3:46

3 Answers 3


What Is This Component?

It's the outer magnetic coil of a reed relay. The two axial connections connected into the glass body are for the reed contact: -

enter image description here

The picture above shows the coil (bottom) and a couple of reed contact options with the top being a SPNO. The lower one looks like a 4PNO device But they could easily be NC types.

Here's an article that shows this explanatory picture: -

enter image description here

Good luck.


It was a reed relay. Perhaps you can purchase a reed capsule to replace the one you broke.

enter image description here

You can also purchase complete reed relays if you know the coil voltage. Check a distributor such as digikey.com for various options.

  • \$\begingroup\$ Good idea! I will see if I can get the rest of it out. \$\endgroup\$
    – Andy
    Mar 27, 2021 at 20:14

That's a reed relay.

It works like a regular relay. Current through the coil causes the contacts to close.

They are usually for low current only.

You'll need to find out what the driving voltage is to get a proper replacement.


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