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The right pull-up resistor

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bison

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pull up resistor relay

Hi and thanks for looking into the attached diagram.

On the left side (green) is a digital downcounter board. The output is open collector, normally high and goes (and stays) low when the counter reaches zero.

On the right side (yellow) is a relay driver board.

The relay must be activated while the counter runs and disactivated when the counter reaches zero.

- Did I choose the right input (rising flank)?
- What is the best value for the pullup resistor Rp? I thought around 1K/.5W, but I'd like to ask you how to find the right value, not by trying but by looking into the diagram.
 

Hi

First we have know the current/resistanece of the relay and the beta of the transistor ,and afther it's
more simple to calculate the resistor.
But if you want you can use the fail input of the relay board so you
did not need broke the head with the resistor (of corse you need invert your control signal).

Eg... i hope that you know what happen with your relay in power up of
the system

Best regards. 8)
 

When relay is ON it draws about 45mA. Transistor has Hfe about 150 for A type. Rp can be about 10kohm. Your input is OK.
 

i hope that you know what happen with your relay in power up of
the system

Thanks for being concerned. The initial idea was to have the relay energized on power up, and de-energized when the counter hits zero (or when 12v supply fails). That in conjunction with using the relay's N/O terminals, so the motor would stop at count zero.

The motor itself originates from a sewing machine and has a speed controlling footpedal with a spring that makes the pedal return to zero when foot pressure is not applied. So the speed controller avoids that the motor starts unexpected when power comes in. Just like a sewing machine : plug it in but the motor will not start until you push the pedal.

The idea to avoid a pullup resistor doesn't look bad either. I'm thinking I could even avoid the use of an inverter and go straight into the fail input, but then use the N/C terminals for the motor. The motor will also stop when the counter reaches zero. The difference would be that the motor now always can work wether the counter+relay boards are powered or not.

Guys I'm gonna try all your suggestions :idea: out :)
 

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