View Full Version here: : 555 timer oscillator for stability for Stepper Motor.

13-05-2017, 09:07 PM
I would like to know if anyone familiar in electronics, can a 555 timer circuit be incorporated into this circuit. Replacing the RC oscillator of R1, VR1 and C5 blue square area (As seen in schematic) with a 555 timer circuit. How would I go about doing that. Thank you and clear skies, Michael

The EQ-mount I am building is near completion, just need to attach the stepper motors and telescope cradle to support the telescope.

13-05-2017, 11:14 PM
It's very easy to do. You can buy 555 timer modules on ebay. http://www.ebay.com/itm/2PCS-5V-12V-NE555-Adjustable-Frequency-Pulse-Generator-Module-For-Arduino-WC-/321962757029?hash=item4af679cba5
Some kits come as a bare board and loose components that need to be soldered. Others are fully assembled. I prefer the unassembled so that I can swap temperature stable capacitors and resistors

Your circuit has both internal and external clock inputs at S2. Leave the timer circuit you have outlined in tact. Cut the track in your existing timer circuit and connect the clock line from the 555 board at the external terminal of S2.

If you replace the capacitor with a temperature compensated cap and replace the two timing resistors with metal film resistors (mfr) you'll get much better thermal stability. NPO/COG ceramic capacitors have excellent thermal stability.

The inboard oscillator roughly seems to be adjustable from 0.14Hz to 14.5Hz. Initially, I suggest you install a 1Mohm pot to adjust the 555 frequency. If your worm is a Byers 10" 360 tooth (hard to tell from the pic) the worm rotation is about 4 mins. Initially calibrate this by timing the revolution of a pointer attached to the stepper motor shaft. Allow for the reduction gear between stepper motor and worm. eventually, fine tune by tracking a star with an illuminated crosshair eyepiece. Adjust the speed then swap in most of the resistance value with an mfr and a small part of the value with an adjustable trimpot then repeat the calibration.

A 555 has 2 timing resistors. The combined resistance determines the frequency of the circuit, the ratio determines the duty cycle, ratio of time on to time off. In your application, the duty cycle is irrelevant. The circuit only needs to provide timing pulses. Short or long pulses are the same to this circuit.

good luck

13-05-2017, 11:46 PM
Just download a 555 data sheet. Has circuits for all types of operation eg
As above good quality capacitor etc needed

But I'm not sure how stable the frequency output is though. You might want a crystal oscillator? But not certain about this

14-05-2017, 10:53 AM

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.



14-05-2017, 03:16 PM
Thank you Joe and Chris, this is greatly appreciated. I'll let you know how I go. Clear skies, Michael

14-05-2017, 04:39 PM
555 is not stable enough for AP.. however it is adequate for visual, and definitely better than oscillator with Schmitt trigger.

Better still, why don't you go for this (http://www.stellarjourney.com/index.php?r=site/equipment_onstep)?

15-05-2017, 07:14 AM
I noticed you were asking about Stellarium, mentioning your Pillow Block mount.....

This program (Stellarium... as well as many others, Cartes du Ciel for example..) can work with OnStep controller (mentioned in my previous post.

If you are going into trouble with building electronics for your mount, do it properly form the start... because sooner or later you will come to this conclusion yourself anyway :P

There are other projects that will control your mount properly:
AstroEQ (https://www.astroeq.co.uk/downloads.php), Mel Bartels stepper design (http://www.bbastrodesigns.com/StepperSystem.html), PIC-GoTo... SoundStepper (http://soundstepper.sourceforge.net/)...

15-05-2017, 07:33 AM