# frequency range too small on 555 timer circuit?

#### AFex54

Apr 10, 2015
144
http://www.electroschematics.com/5834/pulse-generator-with-555/
I breadboarded this 555 pulse generator and the duty cycle adjustment is very good however i cant get the frequency range I would like, it is very limited to the point of only noticing a change in frequency at either extreme of the potentiometer (using an led as visual)
any idea what is responsible for the narrow frequency range?
i tried different ratios of P2 and P1 without improvement.

#### Colin Mitchell

Aug 31, 2014
1,416
C1 is going to have the biggest effect.

#### AFex54

Apr 10, 2015
144
C1 is going to have the biggest effect.
adjusting C1 helped me get into my desired frequency band but had no effect on the width of the band,
I would need a variable capacitor,
maybe 555 timers are designed more for setting to specific freqeucny and leaving it there?
I suppose a variation of caps on a rotary switch would achieve this but im hoping theres an easier way

Last edited:

#### davenn

Moderator
Sep 5, 2009
14,150
it mite be helpful to tell us what freq range you want/need. say from 100Hz to 1000Hz etc

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,508
maybe 555 timers are designed more for setting to specific freqeucny and leaving it there?

Circuits using a 555 having a single control for duty cycle are pretty simple.

Likewise, circuits using a 555 having a single control for frequency are pretty simple.

However, it is not particularly easy to design a 555 circuit where you can independently control duty cycle and frequency. It sounds like this is what you want to do.

#### AFex54

Apr 10, 2015
144
it mite be helpful to tell us what freq range you want/need. say from 100Hz to 1000Hz etc
around 0.2 Hz to 2 Hz with duty cycle from around 10% to 90%

#### AFex54

Apr 10, 2015
144
Circuits using a 555 having a single control for duty cycle are pretty simple.

Likewise, circuits using a 555 having a single control for frequency are pretty simple.

However, it is not particularly easy to design a 555 circuit where you can independently control duty cycle and frequency. It sounds like this is what you want to do.
yeah it is, I guess the rotary switch with caps is my easiest option

#### AFex54

Apr 10, 2015
144
I'd like to add that the width would be fine as just a SPDT switch for 10-20% (typical synthesizer trigger) and 50-60% (typical synthesizer gate)
doubt that makes its any easier though

#### Alec_t

Jul 7, 2015
3,331
Here's how you could get the frequency and duty cycle you want, using half of a six-inverter IC such as the 74HC04.
Frequency (0.2Hz to 2Hz) and duty cycle (2% to 98%) are independently adjustable.

#### AFex54

Apr 10, 2015
144
Here's how you could get the frequency and duty cycle you want, using half of a six-inverter IC such as the 74HC04.
Frequency (0.2Hz to 2Hz) and duty cycle (2% to 98%) are independently adjustable.
View attachment 20876
sounds great and looks surprisingly simple! I'll have to do some reading on inverters first to know whats actually going on

#### Alec_t

Jul 7, 2015
3,331
Here's what's going on :-
R1/R2/U1a/U1b form a large-hysteresis Schmitt trigger circuit with an inverting node at U1a output. C1 and the SetFreq pot then turn this into a square-wave oscillator. The waveform across C1 is approximately triangular and is summed (via R3/R4) with a voltage set by the SetDuty pot. As the sum signal crosses the switching threshold (approximately half the supply voltage) of U1c, so the output of U1c flips from rail to rail to form a PWM waveform.

#### CDRIVE

##### Hauling 10' pipe on a Trek Shift3
May 8, 2012
4,960
Here's how you could get the frequency and duty cycle you want, using half of a six-inverter IC such as the 74HC04.
Frequency (0.2Hz to 2Hz) and duty cycle (2% to 98%) are independently adjustable.
View attachment 20876
Alec, I like it. It's a clever simple circuit but it's giving me a case of deja vu. Have you ever posted that circuit elsewhere?

Chris

#### Alec_t

Jul 7, 2015
3,331
You're probably thinking of my 'Simple PWM' article over at ETO .

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