# 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? 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. 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 Dec 27, 2021 at 13:37
• Are you using solderless breadboard, that has extra capacitance which will result in the wrong frequency. Dec 27, 2021 at 13:52