inverted oscillator?

[mstechca]

It seems that many oscillators are made to satisfy the equation frequency=1/(resistance*capacitance)

Where can I find a one or two transistor oscillator that satisfies the equation frequency=1/((1/resistance)*capacitance)?

The reason why I ask this is because I want to eventually make a light detector. I managed to make the detector detect darkness, but I want it to detect light instead.

SO if I rip apart a two transistor astable multivibrator and configured it so that it detects light, how would I do it?

I have a CDS cell that gives low resistance in bright light and high resistance in the darkness. Where would I place this cell?

 

[Styx]

I am a bit confused?

Why do you need an oscillator for a light sensor?

You say you have a detector that detect "darkness" but darkness is just the lack of light?

What are you using as a "darkness" detector?

All I would do is get a photo-diode and build a trans-impedance amplifier to give me a voltage that is proportional to the light (or lack of light) falling on the photo-detector

what cct are you using?

 

[Nigel Goodwin]

It all sounds very confusing? - why do you want an oscillator?.

As you have a CDS cell, the simplest way is to use an opamp as a comparator, with the CDS cell as one element in a wheatstone bridge. You simply swap the CDS and one resistor over for 'over light' or 'over dark', the output of the opamp switches at a specific level, which can easily be adjusted with a variable resistor.

There are plenty of examples of this on the net.

 

[mstechca]

It is a remote light sensor.

It consists of a transmitter and a receiver.

The receiver is light a normal radio except that the output is shown in an LED instead of a speaker.

The transmitter transmits a tone. The frequency of that tone is based on the amount of light fallong on the CDS cell.

It seems that most oscillators carry a high pitch with lower resistance values. I want the exact opposite of this.

If you still don't understand, please tell me how I can make a long range transmitter using only BJT transistors, resistors, capacitors, and inductors, and an antenna. The transmitter must be able to produce an extremely high pitched sound in darkness (when the resistance of the cell is high) and a low pitched sound in bright light (when the resistance of the cell is low).

 

[Roff]

This is probably more complex than what you are looking for, but I think it will work. I ran some sims on it. You may have to adjust some values, depending on the light and dark resistances of your photocell. It obviously uses more than a simple 2-transistor multivibrator. I'll give some more thought to that, but my first thought was that it would be difficult to do.
If you want to transmit this by RF to a receiver, you'll have to modulate a transmitter with the output.

 

[Roff]

This is a test post to see if this thread is still screwed up. The preview window comes up with no text.

 

[Roff]

Well, the test worked. Let's see if I can post a schematic.
Here's a somewhat simpler version that uses the cross-coupled multivibrator. Darlingtons are needed because of the wide range of current involved, and they are also one way to solve the base-emitter breakdown problem.

Edit:
The op amp needs rail-to-rail I/O capability.