# Astable multivibrator frequency mismatch

I made an astable multivibrator using an NE555p and a transistor. I checked the frequency and found it to be 220 kHz, but it should be 481 kHz.

I used a resistor of 1 kΩ for both R1 and R2. The capacitance value is 1 nF.

The formula I used is 0.693 × (R1 + 2R2) × C.

I made the circuit on a breadboard. I don't know why the frequency is so low.

When I changed the resistor value to R1 = 1 kΩ and R2 = 10 kΩ, the output was 43 kHz but it should be 66 kHz. I don't know why there is such a huge variation in the circuit.

• Does this answer your question? Is the NE555 the IC I need, and if not, what do I replace it with? Commented Dec 27, 2021 at 13:32
• See "Speed" under "systematic problems". Anything above say 100, 150 kHz is really "good luck if it works" with the NE555, depending on how high VCC is. Your schematic also omits the decoupling for VCC and what your VCC actually is. I bet you really don't want to use an NE555 if you need an oscillation above 100 kHz. At the very least, you'd want the (typically much faster) CMOS variants of the 555. But really, in a lot of cases, using a microcontroller is the winning move here: easier to hit the right frequency, easier to adjust, less components, in effect thus cheaper. Commented Dec 27, 2021 at 13:35
• To cite the TI NE555p datasheet ("8.1 Overview", first sentence: The xx555 timer is a popular and easy to use for general purpose timing applications from 10 μs to hours or from < 1mHz to 100 kHz. And 481 kHz > 100 kHz Commented Dec 27, 2021 at 13:37
• Are you using solderless breadboard, that has extra capacitance which will result in the wrong frequency. Commented Dec 27, 2021 at 13:52

## 1 Answer

With the 555 timer I believe there is a normally a discrepancy at higher frequencies between the calculated frequency (using the data sheet equations) and the actual output frequency. This discrepancy increases as the set frequency increases with the calculated frequency always being higher in value than the actual measured output frequency.

I believe this discrepancy is due to the internal delay within the 555. This is the delay between when the voltage on pin 2 (the timing capacitor voltage) crosses either of the thresholds of the two internal comparators and when the transistor, which switches the discharge pin (pin 7), actually switches.

The two threshold voltages of the 555's internal comparators are internally set at about 1/3 Vcc and 2/3 Vcc.

If you have a look at the 'triangular' waveform on pin 2 with an oscilloscope you will see that, at higher frequencies, the 'triangular' wave extends (ramps up) above the 2/3 Vcc threshold level and ramps down below the 1/3 Vcc threshold level. I believe this is not due to the comparator threshold levels changing at higher frequencies but due to the interal 555 delay, the delay between when the pin 2 'triangular' wave crosses a 1/3 Vcc or 2/3 Vcc internal comparator threshold and when the pin 7 discharge pin actually switches.

This internal delay becomes more significant at higher frequencies because at higher frequencies it become more significant compared to the high and low times of the output signal, that is to say compared to the positive going and negative going ramp periods of the pin 2 'triangular' waveform.

So, I believe that because of the internal delay within the 555, the pk to pk amplitude of the pin 2 'triangular' wave increases as frequency increases which results in a discrepancy between calculated and actual frequency which also increases as frequency increases.