defining outputs

[mstechca]

ok, now I go resistive instead of capacitive style.

anyways, I could pull off a circuit which gives me a unique resistance based on a 4-bit binary output.

I'm using a 74HC393 binary counter.

I want to have it so that the output pins of the counter are the same as VCC, or floating (resistance is higher than the highest resistor available in the market).

I could get away with it by adding a diode in series with the resistor, because a diode can't accept small reverse currents.

But is there a way I can do it without the diodes?

I'm doing this because I'm making a digital voltage divider and I don't want any internal resistances (connected to the output pins of the IC) to interfere.

 

[Nigel Goodwin]

ok, now I go resistive instead of capacitive style.

anyways, I could pull off a circuit which gives me a unique resistance based on a 4-bit binary output.

I'm using a 74HC393 binary counter.

I want to have it so that the output pins of the counter are the same as VCC, or floating (resistance is higher than the highest resistor available in the market).

I could get away with it by adding a diode in series with the resistor, because a diode can't accept small reverse currents.

But is there a way I can do it without the diodes?

I'm doing this because I'm making a digital voltage divider and I don't want any internal resistances (connected to the output pins of the IC) to interfere.

You don't need any floating pins, just HIGH or LOW, try googling for R2R ladder, this is a simple digital to analogue converter using just resistors, and should be perfect for your scheme.

However, before you bother doing that, I would suggest perfecting the varicap tuning system - use a potentiometer (or preset resistor) to feed a voltage to it, in this way you can evaluate the voltage range required.

 

[audioguru]

MStechca,
A diode in series with the outputs won't do any good. You need the outputs to drive the resistors both high and low.

Cmos ICs have a higher output resistance at lower supply voltages. Using a 5V regulator at its minimum of 4.5V, a typical 74HCxx output can provide 35mA at 2.25V which calculates to be a 64 ohm resistor. Its minimum current is 12mA so its max resistance is 188 ohms.
All the 74HC4017 ICs I've used in my 6V Ultra-bright Chaser project measure typical or better, that's why they're so darn bright!

If you use 10k resistors as the minimum value in your resistor ladder at the outputs of the counter then the circuit's max error would be only 188/10k= 1.88%. If you use 100k resistors then the error is 0.2%.

If you operate the counter with a 6V supply (its absolute max is 7V) then the error is halved.

The 4-bit counter feeding a resistive ladder would provide 16 voltage steps for your varactor diode.

The 74HC393 has a typical max clock of 99MHz, and maximum rise and fall times of nearly nothing for its clock input. Therefore you better have a 74HCxx Schmitt-trigger inverter as the debounce circuit for your pushbutton. :lol: