Astable Circuit Using 74HC00

Hi,

Can somebody tell me how to compute the R/C combination or this circuit to get the FREQUENCY of oscillation?

**broken link removed**
**broken link removed**



I tried bread boarding this circuit and wonder why some 74xx00 works while others don't??

Any advice will be appreciated..


TIA

You may want to read this link as a starting point. Note the nomograph charts.

I don't quite get the circuit you posted. The link shows some variations.

Circuits like this don't always take well to bread boarding due to stray capacitance and other factors. They also very unstable as to frequency. For a given R/C the frequencies will vary chip to chip and as you noticed some chips will and some won't oscillate.

If you want a stable square wave I would look at using a simple and inexpensive oscillator.

Ron

I have breadboarded things like ring oscillators before and they work... I certainly wouldnt call them the most precise things in the world but they do exhibit oscillatory behavior.

in my experience parasitic capacitance on the cheap-o run-of-the-mill breadboards is somewhere in the vicinity of 50 pF (+/- 40pF...), depending. since you are up in the nF range you should be safe from those parasitics.

it looks like you have 2 oscillators there, one at a low frequency driving one of a higher frequency. I am not sure why you would want such a configuration? unless the two oscillate at exact frequency multiples of one another you are going to get destructive/constructive interference at the middle node. its tough to tell what would actually happen.

try breaking the node in the middle and measuring what each circuit does individually. can you simulate this?

I can't remember old TTL, but when a Cmos gate is used as a two-inverters oscillator, only one input should be used so that the switching threshold voltage is closer to half the supply voltage.
Then the gate can be used properly as a logic gate to gate the higher frequency oscillator on and off.

The first resistor value should be about 10 times higher than the timing resistor value.

High speed Cmos is very powerful so if used as a low frequency oscillator it might overheat while it is a linear amplifier. Therefore a Schmitt-trigger oscillator should be used for low frequencies.

I tried simulating the circuit and I wonder why both are oscillating at 6 mS/cycle...

Your sim software probably knows nothing about a 74HC00 IC.
I have made many oscillators using CD4xxx ordinary Cmos logic ICs and they all work perfectly.
I have made many Schmitt trigger oscillators using 74HCxxx ICs that work perfectly but never this circuit.
I agree that 74HCxxx iCs probably will not work on a breadboard and not without a 0.01uF to 0.1uf ceramic supply bypass capacitor close to the supply pins of the IC.

Hi John,

I'm trying to duplicate the modulated IR emitter for TV remote sensor but I can't make it work...

Hi Rhonn,
Most people use two 555 oscillators or a 556 to create the bursts and pauses of 38kHz required.
You can also use an ordinary CD4xxx Cmos logic circuit to do it.

The 74HCxxx ICs operate so fast that I don't know if they can make an ordinary Cmos oscillator.

Hi John,

Been trying with two 555 or a single 556....wired the first oscillator to run at 38khz but I need the frequency of the second oscillator to modulate the first oscillator..I tried modulating the 38khz with 250hz but the receiver just shutdown after a while...meaning, it wont receive my pulse anymore..

The IR receiver datasheet with pulse link and duration makes me dizzy....


BR

rhonn