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First of all, I want to say I'm a newbie in these kind of electronics. For me to understand things perfectly, I need to understand how things were solved in the old days, where there weren't any microcontrollers, only relays. Nowadays, we can just say, it's all programmed and there are a bunch of transistors etc.

How were DC motors controlled back then, using only relays? The motor I'm interested in is a "pop-up headlights" motor.

These motors had an end stop when the lights were up and an end stop when the lights were down.

We all knowthat if you apply a constant current to the motors and they can't move anywhere (because lights are up and gears/linkage are at their ends) then they just burn out.

How did they manage to make these circuits using only relays? How did they make the current cut off and then apply current again in reverse?

Lights works like this (as far as I know):

  1. Push lights ON/OFF button.
  2. Headlights motors get activated, current flows through them.
  3. Headlights reach their UP position, current is cut off from the motor.
  4. Push light ON/OFF button again.
  5. Headlights motors get activated, current flows through them (assumably polarity is reversed).
  6. Headlights reach their DOWN position, current is cut off from the motor.
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  • \$\begingroup\$ this may be of interest duckduckgo.com/?q=relay+cpu&ia=web \$\endgroup\$
    – jsotola
    Feb 17, 2023 at 21:22
  • \$\begingroup\$ mechanics.stackexchange.com may be a better place to ask \$\endgroup\$
    – jsotola
    Feb 17, 2023 at 21:24
  • \$\begingroup\$ It's not like we went from relays to microcontrollers with nothing in between. There may have been a time where components were limited to relays and a few other things, but in those days there weren't pop-up headlights either. \$\endgroup\$
    – John D
    Feb 17, 2023 at 21:25
  • \$\begingroup\$ Could be limit switches, time-delay relay (probably), or current sensor to detect the jump in current when the motor stalls out. \$\endgroup\$
    – vir
    Feb 17, 2023 at 22:13
  • \$\begingroup\$ This one doesn't use any electronics. Just a hand crank. \$\endgroup\$
    – RussellH
    Feb 18, 2023 at 0:51

2 Answers 2

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Here's how it's done, using a single DPDT ratchet relay to reverse the motor with the push of a button.

enter image description here

The ratchet relay contacts, changing over on actuation of the push button switch, start the motor. Limit switch actuation, at the end of travel, stops the motor.

Re-actuation of the push-button switch reverses the motor till the limit switch is actuated at the other end.

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  • \$\begingroup\$ Where did the diagram come from? \$\endgroup\$
    – JRE
    Feb 18, 2023 at 8:28
  • \$\begingroup\$ It's my own diagram! \$\endgroup\$
    – vu2nan
    Feb 18, 2023 at 8:43
  • \$\begingroup\$ OK. It looked like something from a school book, so I thought I'd ask. \$\endgroup\$
    – JRE
    Feb 18, 2023 at 8:47
  • \$\begingroup\$ Anytime! Thank you. \$\endgroup\$
    – vu2nan
    Feb 18, 2023 at 9:55
  • \$\begingroup\$ This is exacly what i needed to understand this! I'm a bit slow reader so i took it to Paint and started following what does what exactly! Good diagram Is it possible for you to draw similar scheme, but for wiper motors? Which i mean, even when stalk is switched to OFF mid wiping, then it does its rotation and ends up in park position. Thanks! \$\endgroup\$
    – wtknow
    Feb 18, 2023 at 22:36
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Pop up headlights are a subject near and dear to my heart, thank you for asking!

Firstly I'd like to address your claim that "These motors had an end stop" - the good ones are actually on a crankshaft, and if you were to run the motor continuously in any direction the lights will cycle up and down without burning out the motor.

As to control, these motors and other position controls (for instance your wipers - why do they always stop when they're down?) used switches on a camshaft to gate power into the motor. Consider a system where you want a motor to stop when in position A - apply power to that motor through a normally closed switch, and position a cam on the motor shaft such that it opens the switch when at position A. This is how wipers can function.

Here's a snippet from the 1982 Toyota Supra wiring diagram detailing the headlight retract motor: Supra retract motor detail

The annotation gives away the control secrets: "3-2: closed unless retract motor at up position" and "3-6: closed unless retract motor at down position". The retract relay interfaces with this setup to drive the motor into just the right spot.

And in case you were wondering why the motor doesn't overshoot the stop position due to inertia: worm gears! nature’s lossiest gear.

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  • \$\begingroup\$ Interesting terminal number assignment. Disappointingly, no genuine pretzel in the wiring diagram. \$\endgroup\$
    – greybeard
    Feb 18, 2023 at 8:24
  • \$\begingroup\$ Hello, this had made me lot of sense, but only to a certain point. If the camshaft now "Pushes" the switch OPEN. How do i reapply the power to make the motor close the pop up headlighs? Because the circuit, that powered the motor, is now mechanically opened by the camshaft. And for wiper motors, they work till they had made their whole rotation even if i switched to OFF position midway wiping,. How is it solved? Thanks \$\endgroup\$
    – wtknow
    Feb 18, 2023 at 21:20

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