Resistor Problem - Electronic Circuits Projects Diagrams Free

samarsingla · 26th December 2006, 11:27 AM

Hello all,
I am using some push butoons in a robot remot control. The default state of the button is grounded through a 10k resistor and when pushed it is connected to a 5 v source. The output is going to an or gate ic. Now the problem is that in the default state, some current is flowing from the input pin of the or gate through the 10 k resitor and the voltage at the button pin is about 1.5volt. So the pr gate is on in the default state. What is the threshold at which or gate besomes high? How do I correct this? By using pull down resistors of lower value? Please someone guide me.
Thanx

Dean Huster · 26th December 2006, 01:05 PM

Lower the value of your pull-down resistor to lower the value of the voltage at the non-activated input. Sounds like you're using TTL circuitry as CMOS would have worked fine with 10K pull-down.

This is exactly why with TTL that pull-down resistors should be used only as a last resort. The current drawn by an input to pull it down is much higher than that used to pull it up. That same 10K resistor used as a pull-up resistor would work just fine. In TTL, you would be better to wire an inverter between the switch and the IC input and then use a pull-up resistor in the input of the inverter and ground the inverter with the switch to initiate the robotic function you desire.

Dean

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26th December 2006, 11:27 AM	(permalink)
samarsingla	Resistor Problem Hello all, I am using some push butoons in a robot remot control. The default state of the button is grounded through a 10k resistor and when pushed it is connected to a 5 v source. The output is going to an or gate ic. Now the problem is that in the default state, some current is flowing from the input pin of the or gate through the 10 k resitor and the voltage at the button pin is about 1.5volt. So the pr gate is on in the default state. What is the threshold at which or gate besomes high? How do I correct this? By using pull down resistors of lower value? Please someone guide me. Thanx

26th December 2006, 01:05 PM	(permalink)
Dean Huster	Lower the value of your pull-down resistor to lower the value of the voltage at the non-activated input. Sounds like you're using TTL circuitry as CMOS would have worked fine with 10K pull-down. This is exactly why with TTL that pull-down resistors should be used only as a last resort. The current drawn by an input to pull it down is much higher than that used to pull it up. That same 10K resistor used as a pull-up resistor would work just fine. In TTL, you would be better to wire an inverter between the switch and the IC input and then use a pull-up resistor in the input of the inverter and ground the inverter with the switch to initiate the robotic function you desire. Dean __________________ Dean Huster, Electronics Curmudgeon Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines). R.I.P.