DIY a Compact RC Switch

[Chimicron]

Good day~all,



I have made a compact RC switch circuit,which built around the LM358 comparator reacts to a 50Hz PWM signal with a pulse width of around 1-2ms. If the reference voltage ( at pin2 of LM358 ) fixed by the 10K oitebtuineter is lower than the filtered signal voltage (at pin 3 of IC1), the comparator output goes high, relay driver BC547 (T1) is switched on and the 5V relay (RL1) works. The 100K reistor ( R5) provides small hysteresis to prevent the comparator from responding to every minor wavering in the voltage on the non-inverting input (pin 2) of the comparator. The aim I bluit this circuit is to control the on/off of lighting on radio control projects.

Well,Comparing with the above circuit,the only differences are that

1).The relay driver(T1) I used is BC547A instead of BC547. This is the BC547A PDF.

2).My circuit has a 20K pot and it will not work,I am having a hard time figuring out the pin 2 input,giving a negative voltage.



Can anyone here help me?



Best regards~

 

[betwixt]

The circuit should work but the LM358 isn't very good when working from low voltages, using a real comparator would help.

The PWM amplitude will be halved by the divider of R1/R2 so if you are feeding a 5V PWM signal it will never go higher than 2.5V and for example, at 50% PWM ratio will only be 1.25V. Adjusting P1 sets the voltage at which the output at pin 1 should switch state, when the voltage at pin 3 goes higher than the voltage at pin 2, pin 1 should go high and turn the relay on.

I suspect your problem is you are using an op-amp that doesn't work very well at low voltages, you should really use a comparator instead as they are designed to switch abruptly instead of passing through a linear region.

Brian.

 

[KlausST]

Hi,

The description is a bit confusing.
* I see nothing related to RC in your circuit.
* I see nothing related to PWM or 50Hz. I guess it will work with much higher frequency like 10kHz as well as DC input.

I miss an error description:

it will not work

What do you expect it to do? What does it do instead?

Negative voltage? I see no (need for) negative voltage.

Debug: report us the voltages at pins 1, 2, 3 of IC1
What´s the nominal relay coil current?

Improvement: Put a filter / stabilizing capacitor at pin2 of IC1.

Klaus

 

[danadakk]

The input common mode range for this OpAmp is limited -

Then there is the possible phase inversion problem with the LM358 -

Audio guys report unless output loaded with 1 mA load or greater severe crossover distortion.
Translate DC issues when operated as a comparator, eg. non linear response under the right
conditions affect clean desired comparator operation. Maybe a better RRIO OpAmp should
be in the works for the design.....

https://www.ti.com/lit/an/sboa059/sboa059.pdf?ts=1657786891458&ref_url=https%253A%252F%252Fwww.google.com%252F

Regards, Dana.

 

[Chimicron]

The circuit should work but the LM358 isn't very good when working from low voltages, using a real comparator would help.

The PWM amplitude will be halved by the divider of R1/R2 so if you are feeding a 5V PWM signal it will never go higher than 2.5V and for example, at 50% PWM ratio will only be 1.25V. Adjusting P1 sets the voltage at which the output at pin 1 should switch state, when the voltage at pin 3 goes higher than the voltage at pin 2, pin 1 should go high and turn the relay on.

I suspect your problem is you are using an op-amp that doesn't work very well at low voltages, you should really use a comparator instead as they are designed to switch abruptly instead of passing through a linear region.

Brian.

Thanks for your help, I figured out that it should be the BC547 that doesn't work well with more than 50mA.

 

[Chimicron]

The input common mode range for this OpAmp is limited -

View attachment 177402

Then there is the possible phase inversion problem with the LM358 -

View attachment 177403

View attachment 177404


Regards, Dana.

Dana Thanks for your help, I will check the possible problems of LM358

 

[Easy peasy]

Yes 50mA is getting up there for a BC547 if you don't have 3-5mA of base current, i.e. 1k drive.

--- Updated Jul 14, 2022 ---

The inputs of the LM358 can go to gnd ( -0.3V in fact ) but they cannot go near +Vcc as explained above ...

 

[danadakk]

OpAmps as comparators (multiple references at end of article) :

https://www.edn.com/using-an-op-amp-as-a-comparator-here-is-what-you-need-to-know/

Regards, Dana.