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ling1995

DIY a Compact RC Switch

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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. 

 6.2.2.png.3a0e6ef149afa67c6df370c77a95a16a.png

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~

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The circuit will never have a negative voltage in it.

P1 will smoothly give 0V to +5V to pin 2 as it is rotated. The input current of pin 2 is so low that a 10k or 20k pot will make no difference.

R4 limits the T1 base current to  only 0.3mA then tie relay might not be activated if it needs 6mA or more. The current in R4 must be at least 1/20th the current in the relay coil. The BC547 works poorly at currents above 50mA.

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