In the building I live, the hallway lights can be turned on by pressing any button that are scattered around the building, and all the lights will go on for a given time. I'm pretty sure that its totally analog, since it's decades old for sure. What is the working principle of this?

I'm thinking that pressing the switches might charge a capacitor, which slowly discharges through an electromagnet, which keeps the lights on for some time. This would require cables running from all the switches to a capacitor and from there back to all the lamps, which is a lot of cables. Does anyone know how they did these "back in the days" (I'm guessing modern systems would be simply digitalized with some cheap microprocessor).


  • \$\begingroup\$ How old do you think microcontrollers are? \$\endgroup\$ – Ignacio Vazquez-Abrams Oct 18 '17 at 14:53
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    \$\begingroup\$ Related electronics.stackexchange.com/a/334256/139766 \$\endgroup\$ – Trevor_G Oct 18 '17 at 14:56
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    \$\begingroup\$ "BACK IN THE DAYS".. Decades... BOY I feel old... \$\endgroup\$ – Trevor_G Oct 18 '17 at 14:58
  • \$\begingroup\$ @IgnacioVazquez-Abrams The building is over 120 years old, I'd not be surprised that the cabling was not renovated many times, so it could be from the first half of the last century :D Btw, your comment made me think they're older than what I thought, but wikipedia says "The first microprocessor was the 4-bit Intel 4004 released in 1971", so about the time I thought :) \$\endgroup\$ – fbence Oct 19 '17 at 15:34

As discussed in Adding a timer to a basic circuit without using a 555, a time delay can be done pneumatically.

enter image description here

Figure 1. A pneumatic time delay switch.

The simplest solution is to replace your push-button with a pneumatic time delay switch. These are commonly used in stairway lighting applications. The button is pressed to close the contacts and, at the same time, air is pushed out of a piston in the button through a one-way vent. Air is drawn back into the piston when the button is pushed back out by spring. Adjustment of the bleed screw deterimines the time delay.


simulate this circuit – Schematic created using CircuitLab

Figure 2. Typical multi-switch circuit. Put a button and lamp on each floor.

Very simple and reliable.

  • \$\begingroup\$ I already linked that one... \$\endgroup\$ – Trevor_G Oct 18 '17 at 15:39
  • \$\begingroup\$ Good man. None of this electronics stuff. It'll never catch on. \$\endgroup\$ – Transistor Oct 18 '17 at 15:45
  • \$\begingroup\$ :) lol ya it is just a fad. \$\endgroup\$ – Trevor_G Oct 18 '17 at 15:47
  • \$\begingroup\$ I like the ticking noise from the clockwork rotary wind up types too. Especially since they make that extra little noise just before they turn off which kind of acts as a warning. \$\endgroup\$ – Trevor_G Oct 18 '17 at 15:50

Back in the seventies (that's four or five decades ago) we used time delay relays for this sort of thing- this function could be achieved with a "delay timer" function and the NC contact of the relay.

The switches would be wired in series (normally closed) to interrupt the power to the timer.

Normally open switches could be use used in parallel with an auxiliary relay, or an "interval timer" function could be used.

The internal circuitry of the timers used programmable unijunctions and small SCRs with an RC timing circuit- no fancy microcontrollers. For very long delays sometimes 4000 series CMOS.


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