555 timer frequency is way off

A 555 is not really a precision device, and as you've found, if the resistor values or the capacitance value is too low you'll see relatively large deviations from the theoretical values, especially with the old bipolar type operated from 5V. Stray capacitance affects a 100pF capacitance value, and 150 ohms is low. Bypassing pin 5 has an effect on the frequency that is not covered by the equations. Typically the frequency is lowered by as much as 10% depending on how good your bypassing is.

You'll probably do better with a 1nF NP0 capacitor and the CMOS version of the 555. Consider feeding back the voltage from pin 3 with a single resistor rather than using the discharge pin which forces a rather low value for R48 to get close to 50% duty cycle. If you stay with the bipolar version, in particular, increase the supply bypass capacitor C10 to at least 1uF and reduce the pin 5 bypass to 10nF. The bipolar version draws rather nasty current spikes when switching.


Optical remote receivers such as IR Rx from Vishay/Sharp usually have AGC with a BPF and a Q=10 so you may want to be within 10% tolerance over temp to optimize gain.

Consider a 38kHz XTAL chip $2 and CMOS inverter + passives https://www.digikey.ca/product-detail/en/abracon-llc/ABS25-38.000KHZ-T/535-10243-1-ND/2218056

The reasons for unpredictable k factors of f=k/RC are due to non-square wave cap current in loop. (See my other recent answers why hysteresis ratio matters.) This is why I chose to never use a 555 in my long career when accuracy mattered.

Also is your 5% ceramic NP0/CoG? That matters too.


As a point of reference an LTSPICE simulation of the 555 (at a transistor level) produced the following output frequency for your RC parameters. Seems that this simulation didn't manage to oscillate with that low-value reset resistor of 150 ohms as Spehro has suggested. It is always possible that this 555 model was developed to test other modes rather than astable oscillator.
Stray capacitance not included. Pin 5 bypassed to ground with 0.1uf:

C=100pf, R(pin 7) 1000, R(pin 2,6) 191k, F(pin3)=34.203 kHz
C=10nf,  R(pin 7)  150, R(pin 2,6) 1.82k, no oscillation
C=10nf,  R(pin 7) 10k,  R(pin 2,6) 10k, F(pin3)=4680 Hz
C=10nf,  R(pin 7) 1.82k, R(pin 2,6) 1.82k, F([pin3)=24.899 kHz

C=1.0nf, R(pin 7) 1000, R(pin 2,6) 18.2k, F(pin3)=35.697 kHz