# Astable 555 Circuit Problem

#### Matt A

Aug 28, 2015
2
Okay, I am looking for some insight here and I'm hoping that I can find an answer. Basically, I am attempting to make a chirp (or beep) generator with a 555 timer in astable. I've set up my circuit (see below) and was trying to get a timed chirp from the Piezo (similar to a smoke alarm chirp), but whenever I apply voltage to the circuit, I get a constant ring from the piezo instead of a timed oscillation. Is there something that I'm missing that is giving me the issues or what? I am fairly new building circuits and figured that this might be a nice/small project to start with.

Ps. I am using a TI 555cn IC and not the TS555IN (I didn't rename the IC in Altium when I sketched out the schematic; not even sure if it matters really).

Any help would be much appreciated. Thank you.

#### Bluejets

Oct 5, 2014
6,196
you will need 2 555 chips.(or a 556)other possibilities also.(one to create an oscillator)
The way it is now, the output switches hard on and off.

#### Matt A

Aug 28, 2015
2
Okay, I thought the resistors in the circuit would modify the length of time between on/off and the duration of on/off. Is this correct or have I been misled?

#### Alec_t

Jul 7, 2015
3,331
Is there something that I'm missing that is giving me the issues or what?
Unless C1 has very low leakage its leakage current may well exceed the charging current (< about 1uA) via R1, so C1 will never charge much, so the beeper will continue sounding.

##### Ash
Feb 9, 2013
424
Try changing your 4.7M ohm resistor to 4.7k. Your cycle period seems to me over 300 seconds long, which is why your hearing a continuous beep. Changing the capacitor had the biggest effect on the time period.. If you just changed your capacitor to 220 micro farads, your buzzer will beep every 1 second.

You can change c1, r1 and r2 to get the desired time period. You can use an online astable mode calculator for thishttp://www.ohmslawcalculator.com/555-astable-calculator

#### CDRIVE

##### Hauling 10' pipe on a Trek Shift3
May 8, 2012
4,960
Also insure that the cap is correctly polarized. IE: Neg band is connected to (-) GND.

Chris

#### TedA

Sep 26, 2011
156
Matt,

A few questions for you.

You said " I am using a TI 555cn IC ". TI use the suffix "CN" to define the grade and package. Just 555CN means a bipolar part, such as the LM555CN. You might want to use a CMOS version, such as the TI LMC555CN, or TLC555CP.

The CMOS versions draw much lower currents at the trigger and threshold inputs. This is important when you are using large timing resistor values.

I think there is an extended essay on the forum about using 555 timers for long delays. You might look for it. In short, as the delay becomes longer, the accuracy, repeatability and reliability decrease. You may have to adjust R1 to get the desired timing, and should expect the time to vary over temperature and time.

Is it correct that your piezo device is a beeper and not just a transducer? At least one reply concerns driving a passive transducer.

Was your intent to have the beeper sound for several minutes ( or many minutes, depending on the leakage currents ), then be quiet for a fraction of a second, then repeat? I suspect that the on-time is so long, and the off-time so brief, that you might not even notice that the beep is interrupted by brief pauses.

If you actually want short chirps with a long delay between, you can connect the piezo beeper to Vcc instead of GND.

Ted

#### Colin Mitchell

Aug 31, 2014
1,416
Does you beeper beep when connected to a battery?

Jun 10, 2015
2,699
R1 = 33K
R2 = 10K
C1 = 47 uF

ak

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