I want to turn a load like light on for a certain time when one of elevator buttons pressed say button 1 and while the button pressed the load should be on.

In the figure below circuit 1 describe how the controller of the elevator read button .

I tried two circuits to get low signal to trigger 555 timer .

I tried circuit 2 with 10k for R1 and R2 and i faced a problem . *the led of pc817 always on an this tell the controller the button always pressed.

So i tried to change R1 and R2 to 1M the problem disappeared. But another problem happened when the motor of elevator start this trigger the 555 timer :( I am using decoupling caps 0.1u and 10u.

So i tried to add C1 to trigger pin with different values 0.1u ,1u,10u,47u and 100u the problem still exist.

So i went to try the circuit 3. The problem when the elevator motor start run still exist . So i tried to add 1 uf cap to trigger pin the problem disappeared.

Does the last circuit i used practical circuit and will work properly? is there another approach better than what i tried do what i need?


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You asked if there is a better approach. Based on Circuit #1, you have an MCU at your disposal. Why not use the MCU to do the timing and control of the LED.

  • \$\begingroup\$ Because the controller board of elevator is ready made. So i cant modify the controller program 🤔 \$\endgroup\$ – lolo Oct 16 '19 at 21:11

The original designers realized the motor would produce lots of noise and they already solved the problem for you. What you can do is to duplicate the circuit that interfaces the button to the existing MCU, but connect your circuit to the 555.

Here’s how:

Use a second PC817 in parallel to the first one (with its own 4.7k resistor). Connect the LED side to the button in the same way, and connect the transistor side to a resistor to +5V and the other side to ground. Connect the collector side to the trigger pin on the 555. I would put a resistor between the 555 and PC817, though that may not be necessary. Then you may want to keep the cap on the trigger pin, too. (Depends on the noise on the 5V supply). Power your 555 of that same +5V supply. If that’s not possible, you may be able to use +12V, but use an LDO and use massive caps (you may need 1000’s of uF). Once it works, you can try smaller cap values.


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