Pull Down resistor selection ? - Electronic Circuits Projects Diagrams Free

2camjohn · 1st January 2005, 08:00 PM

Im not sure whether i should post this in the microcontrollers section, either way Nigel Goodwin is gonna answer and thats all i really need :wink: :wink: :wink:

I have a PIC circuit which runs off a 9V battery to control a powerful motor.

I have two inputs which are both connected to the interrupt pin. One of the inputs is external and I am detecting the rising edge of this signal, the other is from a switch on the board with a weak pull down resistor.
They are connected to the same interrupt input using diodes.

The problem is when the battery is very low the switch will not go high enough to cause an interrupt.

The pull down resistor is 100k ohm.

Can I change the value of the pull down resistor to stop this from happening.

I am already using schotty diodes to reduce the voltage drop.

Cheers
John

Juglenaut · 2nd January 2005, 07:08 AM

insufficient pull down current at lower level voltages.

I would use a set of transistors and switch them off and on, that way current is more controllable. Can even go into using op-amps as comparators so when the battery hits a certain voltage it will switch to a different transistor set up for lower voltages, and at the coupling node to the input use two diodes to keep the other higher voltage transitor from leaking current. For sure the first transistor would be a common emitter NPN and the second might be a transistor and/or low power fet that needs a low to turn on.

It is up to you, but I figure being able to set the state of the circuit more actively would yield a better response.

At what voltage does this start to happen??

Just a thought:
BEC, or battery elimination circuitry is incorporated into most small signal receivers in cooperation from a ESC Electronic speed control, which would work in a way so that even if the battery has dumped the receiver still has enough power to run servos without an external receiver battery and cut the power going to the main motor. This circuitry switches states as the battery gets more discharged.

**Nigel Goodwin** · 2nd January 2005, 02:27 PM

I'd like to see the actual circuit, I don't see why taking an I/O pin directly to +5V wouldn't work under any conditions that the PIC would work under. The pull-down resistor wouldn't really make any difference, zero ohms (well nearly - from a switch) and 100Kohm isn't really any comparison.

2camjohn · 3rd January 2005, 07:00 PM

Its working fine now, even with really dead batteries.

I think it must have been a problem with my soldering, somewhere between switch and input pin

ops:

ops:

Thanks for the help anyway guys...

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1st January 2005, 08:00 PM	#1
2camjohn	Pull Down resistor selection ? Im not sure whether i should post this in the microcontrollers section, either way Nigel Goodwin is gonna answer and thats all i really need :wink: :wink: :wink: I have a PIC circuit which runs off a 9V battery to control a powerful motor. I have two inputs which are both connected to the interrupt pin. One of the inputs is external and I am detecting the rising edge of this signal, the other is from a switch on the board with a weak pull down resistor. They are connected to the same interrupt input using diodes. The problem is when the battery is very low the switch will not go high enough to cause an interrupt. The pull down resistor is 100k ohm. Can I change the value of the pull down resistor to stop this from happening. I am already using schotty diodes to reduce the voltage drop. Cheers John __________________ The first rule of mstechca club is do not talk about mstechca club. http://www.electro-tech-online.com/s...induser&u=1245

2nd January 2005, 07:08 AM	#2
Juglenaut	insufficient pull down current at lower level voltages. I would use a set of transistors and switch them off and on, that way current is more controllable. Can even go into using op-amps as comparators so when the battery hits a certain voltage it will switch to a different transistor set up for lower voltages, and at the coupling node to the input use two diodes to keep the other higher voltage transitor from leaking current. For sure the first transistor would be a common emitter NPN and the second might be a transistor and/or low power fet that needs a low to turn on. It is up to you, but I figure being able to set the state of the circuit more actively would yield a better response. At what voltage does this start to happen?? Just a thought: BEC, or battery elimination circuitry is incorporated into most small signal receivers in cooperation from a ESC Electronic speed control, which would work in a way so that even if the battery has dumped the receiver still has enough power to run servos without an external receiver battery and cut the power going to the main motor. This circuitry switches states as the battery gets more discharged. __________________ Your website has been blocked

2nd January 2005, 02:27 PM	#3
Nigel Goodwin	I'd like to see the actual circuit, I don't see why taking an I/O pin directly to +5V wouldn't work under any conditions that the PIC would work under. The pull-down resistor wouldn't really make any difference, zero ohms (well nearly - from a switch) and 100Kohm isn't really any comparison. __________________ PIC programmer software, and PIC Tutorials at: http://www.winpicprog.co.uk

3rd January 2005, 07:00 PM	#4
2camjohn	Its working fine now, even with really dead batteries. I think it must have been a problem with my soldering, somewhere between switch and input pin ops: ops: ops: ops: Thanks for the help anyway guys... __________________ The first rule of mstechca club is do not talk about mstechca club. http://www.electro-tech-online.com/s...induser&u=1245