I agree, but the TS said simple, compact and easy, which as we all know != temperature or voltage stable. But good quality low-TC resistors, and a class 1 ceramic C0G/NP0 dielectric capacitor will cover the worst effects and is probably good enough for this (unspecified) environment & the 555 timing is pretty reasonable wrt voltage.hi Irving,
Just for ref ONLY.
Problem is these types of PG's are very voltage and temperature dependant.
https://gyazo.com/2a502042989cf22e508f2fd1dcbfc65dBut that won't give a 10% duty cycle... this version might... (though I agree the TS did, confusingly, ask for a square wave, not a pulse...)
View attachment 239197
But the ubiquitous 555 still is the best option... and 1 less part! (and yes, I know its 90% not 10% duty cycle, but that can be fixed by swapping pins 3 and 7 and adding a pull-up resistor to pin 7, or just using an inverter on the output.)
R1 = 56k, R2 = 6k8, C = 100nF
View attachment 239199
I realized when building the circuit that I didn't have the transistor that I was planning to use. I tried swapping pins 3 and 7 and adding a pull-up to pin 7. But it seems like I'm getting a 50% duty cycle. Is there something that I haven't considered?As the output inverter? Yes, as your simulation shows... (what simulator is that?)
You're welcome.Proteus, first time working with this simulator. I will build the circuit on a breadboard and try it out. Thank you for your help Irving.
Ah, my bad, it does invert the signal but has the 'side-effect' of forcing the duty cycle to 50% (which is the only way to get 50% duty cycle on a 555). I'd forgotten that!I realized when building the circuit that I didn't have the transistor that I was planning to use. I tried swapping pins 3 and 7 and adding a pull-up to pin 7. But it seems like I'm getting a 50% duty cycle. Is there something that I haven't considered?
With the circuit straight off the datasheet, the positive half-cycle pulse width of a 555 astable multivibrator cannot be less than 50% of the output period. There is a modified version that uses external diodes to produce shorter pulse widths. It is very similar to the NAND gate oscillator in the same post.I'm getting a 50% duty cycle. Is there something that I haven't considered?
That's an option too, though I've found with small capacitor and large resistor values the charging current is quite small and, on the breadboard, the diode characteristics can impact on the on/off times, which rarely shows up in simulations. This is particularly so with lower Vcc (as thresholds are 1.66/3.33v at Vcc = 5v) though with 12V it should be OK (though don't recall trying it for real).Below is the LTspice simulation of an example 555 circuit, using a diode to give a 10% duty-cycle:
Admittedly the effect is minimal in this non-critical scenario, about 1% of the pulse width between 15 and 30degC on 12v, 2% on 5v, but I've seen worse where it did become a problem, lower volts, higher temperature spread and smaller pulse widths.I would think there would be less than a 100mV differences between diodes which, will have only a small effect on the on-time capacitor charging current/time.
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by Jake Hertz