Looking for oscillator and frequency divider schematics

[Erudhalion]

Update: I have built the first oscillator, works fine, I've tested it up to 20kHz, no probs. The wave form looks good on xoscope, except it's hard to tell how square it is at high frequencies, but that shouldn't matter much.

On the other hand, I can't get the frequency divider to work, I've tried both the original circuit and the modified vertion suggested by Kris, but I get no output, and probing around it looks like the signal doesn't get past the diodes for some reason. I'm running it off the same 9V power supply the oscillator is using.

I've done some tests, and for some reason, if I simply connect a diode to the output of the oscillator, I get a distorted square wave which still rises above and below 0, but has a slighly smaller amplitude. I was expecting an asimmetric wave, only above or below 0, depending on which way round the diode is. What am I missing?

The diodes should be 1N4148s.

 

[duke37]

9V supply may exceed the permitted e/b voltage in the dividers. I do not know what the effect of the e/b junctions acting as zeners will be.
Try dropping the supply to 5V.

The Livingston Monarch organ used pnp junction transitors wirh permitted e/b voltage of 25V? It was run on 12V.

 

[Erudhalion]

I'll try, but I'm using the same transistors (2N2222s) in the oscillator, and they appear to work fine there.

EDIT: I've tried various voltages, but all I get at most is a very weak and extremely distorted signal, which presumably is coming through the resistror between the collector and the input. There is still no signal through the diodes.

 

[KrisBlueNZ]

I've never actually used the bistable multivibrator with steering diodes myself, so I can only be of limited help here.

There are many good explanations of the circuit available using http://www.google.com/search?q=bistable+multivibrator+steering+diodes
Some of those designs use PNPs; that's only because they're old. There's no specific reason to use PNPs instead of NPNs. The PNP circuits should still work, but you need to reverse the polarities of the voltages and reverse the direction of the diodes and polarised capacitors to convert the circuit to use NPNs.

Can you post a schematic of the EXACT design you're using, including all details even if they don't seem to be relevant.

 

[Erudhalion]

I have tried both the exact scematic I posted the link to, and the one attached. In both cases the positive supply voltage was 9V, the transistors PN2222As and the diodes 1N4148s.

In the scematic I posted:

R1, R2: 100kOhm
R3, R4: 47kOhm
R5, R6: 3,9kOhm
C1, C2: 100pF

I tried assembling the circuit on a breadboard and on a perfboard and it made no difference. I tried varying the supply voltage from 9V to about 2,3V first only of the frequency divider, then of both the divider and the oscillator, and finally I inserted a potential divider between the oscillator and the divider, but none of these changes had any influence on the circuit, apparently.

By probing with a software oscilloscope, it appears that the signal is practically unaltered before the input capacitors, in then very heavily distorted by the capacitors, and its amplitude reduced, as expected. The diodes, as already mentioned, appear to block it completely, so the only signal getting to the output is that through resistor R2, which is at the same frequency as the input, but much weaker.

 

[duke37]

I first met a bistable in a Clyne organ (valves) 60 years ago. The trouble then was that two resistors were not matched. More recently the Monarch had a tendency of dividing by three, I forget what solved that.

Mullard "Reference Manual of Transistor circuits" 1961
"For effective triggering the time constant (R1 and C1) needs to be about five times the trigger width".

Without capacitors across R3 and R4 there will be no reverse e/b voltage so 9V supply should be fine.

 

[KrisBlueNZ]

The trigger pulse needs to have nice clean edges. If it doesn't, add a common emitter stage to buffer it. It will also invert, it, but that doesn't matter. Use an NPN with its emitter grounded, base connected to 100k to the oscillator's output and 22k to ground, and collector through 4k7 to VCC. The signal at the collector will be a clean sharp shape.

 

[Erudhalion]

I've finally found time to mess with the circuits.

Buffering makes no difference.
As far as I can tell, the problem is that the diodes appear to block the entire signal: its a sharp square wave before the caps, then the caps turn it into a series of pulses, and there is no measurable signal at the base of the transistors. At the output of the divider, I get at most a heavily distorted signal at the same frequency as the input.

Could this be to do with the fact that as the signal is between 0 and 9V, rather than +4,5V and -4,5V the diodes are never reverse biased (or always, depending on which way round they are)?

 

[KrisBlueNZ]

The caps are _supposed_ to turn the edges into pulses. There should be a detectable signal at the bases of the transistors, although the pulse will be short. I'm not sure what to suggest.

I've run a simulation. Here's the result. I've attached a ZIP file containing the LTSpice source file in case you want to run the simulation yourself. LTSpice is a free download from http://www.linear.com.

The "power-up circuit" isn't actually required to guarantee proper simulation but I left it there because it ensures that the circuit starts up in the same state each time. You don't need to duplicate it on your breadboard.

 

[Erudhalion]

Thanks!

I know the caps should turn the signal into pulses, I was just confirming that thing were ok up to that point.

I notice the circuit you simulated has a pair of extra diodes connected to the bases of the transistors, what are they for?

 

[KrisBlueNZ]

The diodes in series with the bases are to prevent possible damage to the transistors during the short negative pulses, where the base may be driven more than about 6~7V negative with respect to the emitter (because the power supply is 9V).

The base-emitter junction of a bipolar transistor behaves like a zener diode, with a voltage of about 6~7V. The transistor can be damaged if current flows into the base in the wrong direction.

 

[Erudhalion]

Done it!
I'm not entirely sure what was wrong, but I rebuilt the circuit testing the whole thing very carefully, and now it works.

Next step is trying to assemble it on a perfboard, and build an whole tone generator: 1 oscillator and 7-8 frequency dividers, depending on the design, which I will now deicde on properly.