Quote:
Originally Posted by OzEclipse
Chris,
You are correct that a crystal oscillator is more stable than a 555. Michael asked about a 555 circuit. Keep in mind Michael is not an electronics expert. I tried to give him a relatively simple and usable solution. Also, I assumed he is not planning to do astrophotography because his circuit has no autoguider input although an autoguider widget could be built to take an autoguider signal and feed an autoguided clock pulse into the external clock function.
When I said a 555 with an NPO/COG ceramic capacitors & MFR have excellent thermal stability, I was referring to that configuration compared to the current relaxation oscillator that is built into his circuit. Quartz crustal oscillators operate at high frequencies - 2Mhz and up. An analogue circuit needs a large divider circuit or a microprocessor taking the crystal clocked signal and dividing down.
A crystal oscillator will be more stable but not adjustable unless a complex programmable or dial up analog divider is installed. If you want an adjustable crystal clocked circuit, the circuit gets more complicated.
In 1983, I designed and built a crystal clocked and fixed ratio analogue divided oscillator. It fed an inverter which generated a 240V 49.86111Hz. This drove a 50Hz synchronous motor on my scope at siderial rate. Beautifully stable, the object remained centered for hours but the worm PE meant that constant corrections needed to be applied.
I have a 555 circuit modified with NPO cap and MFR's. I use it to drive my home built portable solar eclipse mount. The image stays centered for hours if the polar alignment is up to the job even during 10 degree temperature changes. It is also very easy to install switched timing resistors on a 555 circuit to give fast/slow guide or slew controls.
regards
Joe
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Thank you Joe and Chris, this is greatly appreciated. I'll let you know how I go. Clear skies, Michael