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I'm trying connect the 555 timer as a pulse wave modulator. Notice from the diagram below, the - input of Comp1 is internally connected to \$2/3V_{CC}\$. The control voltage is applied at pin 5. The question is how can you apply a varying voltage to a point that is already set to a particular voltage. Doesn't that result in a short ?

Thanks

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You don't quite have it right. Pin 5 is not SET at Vcc*2/3. It is connected to a voltage divider that outputs Vcc*2/3 when there is no other connection. When connected to a voltage source, the currents through all the 5K resistors will change such that pin 5 is now tied at Vcontrol

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No. Pin 5 is connected to one of the junctions in a string of three 5K resistors (and the input of a comparator).

As long as your external voltage source has an impedance that is significantly lower than about (5k || (5k + 5k)) or about 3.33K, you will be able to drive the voltage to whatever you want.

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