Can I use a 7555 with a crystal

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Arjune

Joined Jan 6, 2018
354
I was wondering if I can create an astable oscillator with a 7555 timer and a 32,768 hertz Crystal. This could be an obvious thought but is it possible, especially simply possible? I would eventually divide it down to one Hertz with a digital counter IC to clock a clock.
 

Ian0

Joined Aug 7, 2020
13,158
Unlikely to work. The Schmitt trigger action of the 7555 will probably prevent it working.
Why don't you use a 4060? It has counter and oscillator in one IC, and it works with a crystal, and will get to 2Hz.
Then a 4013 will divide by 2 to give 1Hz.
 

DickCappels

Joined Aug 21, 2008
10,660
Here is the way I get a 32768 Hz clock:
1676929934034.png

Cheaper than a 75555 circuit and it works!
Edit: The "0 uf" capacitor on the left may be anything from 0 to 30 pf.
To wit:
1676930802293.png
 
Last edited:

Papabravo

Joined Feb 24, 2006
22,083
I was wondering if I can create an astable oscillator with a 7555 timer and a 32,768 hertz Crystal. This could be an obvious thought but is it possible, especially simply possible? I would eventually divide it down to one Hertz with a digital counter IC to clock a clock.
It might be difficult. A crystal requires an inverting amplifier and there is no such part internally in any 555 device. A look at the internal functional block diagram might be helpful to ponder.
1676929854649.png
The internal components are two comparators, an inverter, and an RS flip-flop along with a resistor divider network. There may be an analog guru more clever than me, but if a method exists, it certainly escapes me.
 

Ian0

Joined Aug 7, 2020
13,158
Recommended load caps for 32.768kHz crystals are around 10pF to 15pF, so try 22pF in your circuits (as the capacitors appear in series to the crystal).
The crystal will give you almost-but-not-quite-180° phase shift at resonance, but you need some more phase shift to get it to 180° to make it oscillate, and that's what the 270k does in conjunction with the nearest capacitor. That resistor is normally 0 to 1k in a MHz crystal oscillator, but it need to be rather more in a 32.768kHz.
The feedback resistor is not so important - I use 10MΩ. Anything above 1MΩ seems to work.
 
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