All around us, there is a weak electromagnetic energy that can be turned into weak signals/voltages. Stray signals are usually an oscillating higher-lower voltage in relationship to circuit 0V reference ground. These signals can be strong enough for transistor based amplifier inputs that are floating (not connected to any part of the rest of the circuit), to detect them, and respond to them.

Floating (not electrically connected) input pins/terminals pick up stray signals, especially high impedance ones.

10,000Ω is a common pull up resistor value.

Pull down resistor

Pull up resistor for PNP BJT base

PNP bipolar junction transistors are completely off when the base voltage is the same as the emitter. They start conducting, at least a little, when a more negative voltage of about 0.6V is applied to the base in relationship to the emitter. Current is then allowed to flow.

To make sure that a PNP transistor stays off until it gets a strong signal, it’s best to make sure there is always resistance connected from the base to the positive side of the power supply. The resistor that is used to do so is called the pull up resistor.

Since bipolar junction transistors depend on base current, you actually have to put the pull down resistor on the other side of the base’s current limiting resistor. That is shown in the schematic diagram above. Closing the mechanical switch provides a direct connection to the positive side of the power supply, during which time, the base doesn’t sense the pull down resistor at all.

High impedance inputs

MOSFET gates, and most integrated circuit (IC) inputs, do not let current flow through them. They just look at the voltage applied to them.

There should always be a pull up/down resistor connected to the supply rail opposite of the signal you are waiting for. Since the gate/inputs draw virtually no current, you can connect them directly to the opposite supply rail of the one that you connected the pull up or pull down resistor to.

Bistable mode 555 timer circuit uses 2 pull up resistors

555 timer pins 2 (trigger) and 4 (reset) are high impedance inputs that respond only to a low input. 1/3 or less of the supply voltage for the trigger, and probably somewhere around 1/2 the supply or less for the reset pin. Pull up resistors prevent them from doing anything until a full signal is given. A good demonstration circuit of this is the bistable mode/flip flop 555 timer circuit.

Basic 555 timer bistable flip flop mode circuits

Open collector/open drain output

LM393 Inverting Comparator Demonstration Circuit Trimpot Controlled by Electronzapdotcom

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Another use for a pull up resistor is the open collector/open drain output of some integrated circuits. The pull up resistor connected to the positive rail (Vcc) provides a signal, or even power, to an input or load. The open collector output of the integrated circuit is either not conducting at all (considered as being high thanks to the pull up resistor), or it is connected to ground (low output).

In other words, while an open collector output is low, it takes away the power/voltage from a load, or input of a different IC, that is provided by the pull up resistor. It is just like an off switch while high.

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