MRE Zero Speed Switch
I recently purchased a used lathe which originally implemented a mercury filled zero speed switch on the motor shaft pulley. The switch was non-functional, partially missing, and had been bypassed with a manual pushbutton. I discovered that the original switch was no longer available and various schemes and circuits had been devised to replace it but I found the problem of designing and building my own zero speed switch interesting and decided to take on the challenge as a project.
I determined that all that was basically needed was a timer circuit that would be continuously reset by some type of sensor. After some thought I decided a Hall effect gear tooth sensor was the simplest choice for me and my research led me to the Allegro line of products. I used an old Radio Shack Engineer's Notebook for the basic NE555 timer circuit known as a "missing pulse detector" and integrated that design with others from the web. Some thought and experimentation yielded a suitable way to interface the sensor to the timer and control an output relay.
The circuit presented on this page is the result of my project. It works very nicely on my machine. I do want to say here that this is presented for informational purposes only. I am posting it for those who might be interested in designing other circuits based on these relatively new line of sensors. There are many commercial alternatives available for zero speed sensing and this design should not be used as substitute for a manufacturer's recommended replacement part and it certainly should not be used where failure could result in damage to equipment or personnel.
The notes below this schematic diagram offer more details of the circuit.
SCHEMATIC DIAGRAM OF MY ZERO SPEED SWITCH:
Notes:
Sensor: The Hall effect sensor is an Allegro ATS667LSG which produces a square wave signal when in proximity to a moving gear or sprocket tooth. For more information on this sensor click the linked name above. The sensor is available for around $7.00 from DigiKey.
Connections: The #13 and #14 are where the original zero speed switch connected to my machine and "make" when the gear tooth speed is zero. #2 and #5 is the fused and switched 120VAC output from the machine's transformer. I also used an inline 0.1 amp fuse (not shown) in series with the terminal #5 wire.
Power supply: My onboard supply uses a small Radio Shack 12.6V power transformer #273-1385, bridge rectifier #276-1152, and regulator #276-1778. I adjusted the regulator output voltage so that the relay had 12VDC across it when energized by the timer. This worked out to be about 13.1V with my relay. Do not exceed 15V or the timer will fry! I glued the pot (VR1) in position once adjusted. One could also use a fixed resistor. As always, watch the polarity of your electrolytic capacitors and use voltage ratings equal or greater than shown on the schematic. This applies to the entire circuit.
Relay: I used a pc board relay removed from a board in my junk box. The coil pulls 35ma. @ 12VDC. The relay must be able to switch 125VAC with the coil current of the machine starter. RS sells a relay which should work fine. #275-248
R1 & C1: These values control the delay time and are critical to insure that the shaft motion fully stops. The values in the circuit above were determined experimentally and thus far have proven satisfactory. This is for a 60 tooth gear/sprocket. If you change the gear, adjust the delay! A 1 meg pot could be used as R1 to make the delay adjustable.
47K resistor: The 47K resistor that goes from (+) to pin 2 of the timer effects the sensitivity of the circuit. I discovered that with a value of 10K the timer would not always trigger and the relay remained closed. I elected to use the 47K value which is very sensitive and have not had any problems with false triggering or missed triggering. I include this note so the reader will know what to change if triggering becomes an issue with your installation. If you change this value BEWARE of missed triggering. Use as high a value as possible.
External LED: This is a Radio Shack #276-271 12V green LED with holder that I mounted on top of the machine electrical box so that it is visible to the operator. This was an afterthought but it gives you a monitor on the health of the zero speed circuit. Basically if it is on it means that the relay is open. This means that the motor shaft is moving and being detected. This is not a great design on my part since the LED depends on the 555 being healthy. If pin 3 of the 555 is open (not low or high) the LED will glow dimly due to the current going through the relay coil. With a sensitive relay and a high current LED it could actually energize the relay. I tested this failure mode on my circuit by removing the 555 timer (I used a socket) and found that the current was not high enough to energize my relay. This being the case, I decided the external LED was a valuable addition to the system. You other hackers out there might want to improve the indicator LED design. An easy fix would be to use a red LED and connect it in reverse of the drawing and to the (+) supply instead of ground. The LED would then glow red when the relay was closed. I prefer the green myself because it glows when the motor is running and tells me that the relay is open. On my application it works as a "safe to shift" light.
Cable & Sensor: I bought a 25 foot piece of shielded 110 ohm stereo audio cable from a seller on eBay. This is the nice tough rubber covered stuff used to connect guitars to amplifiers. Red is (+), white is signal, and the shield is (-). I soldered this to the sensor and bypass capacitor and covered the joint with silicone rubber compound. The cable was pulled through the existing conduit into the control panel and connected to my electronics. You need a shielded cable and you need to seal all connections on the sensor end. The sensor was attached to a mount fashioned to slip over the existing 3/8" rod on the machine and sense a 60 tooth #25 chain sprocket machined to mount on the motor pulley. The existing swivel mount that holds the 3/8" rod, once in position, was tack welded to the machine leg to prevent it from turning since I found that vibration of the machine was causing the sensor to move down and contact the sprocket.
Circuit Description: This is basically a "missing pulse detector" based on a 555 timer chip. As long as the sensor produces a triggering signal fast enough the output from the 555 remains high and the relay is not energized. If the pulses come too slow, the time expires and the timer output goes low which causes the relay to close. This circuit was derived from the many 555 circuits on the Internet as well as an old Radio Shack Engineer's Notebook. It has simply been adapted for the required timing and sensor interfacing. The ATS667 sensor can stop in either a high or low state which is why it must be capacitively coupled to the timer through Q1. Low pulses trigger the 555 timer and discharge C1 keeping it from timing out.
The Schematic: I want to take the time to plug a product. I have no connection to this company so this plug is from the hacker heart. I drew the schematic for this thing like I usually do, on a scrap of paper in the shop. It looked, as usual, like a 3rd grader on drugs drew it! I went in search of some software that would allow me to draw a decent, presentable schematic to stick in my machine manual. I wanted some CAD software under one million dollars in price that didn't require the rest of my natural life to learn. I found something great! ExpressPCB It is FREE. It is easy to use. They can make you a PC board at a reasonable price. I don't care if you are an electronics tinkerer like me or a professional, you need this. Check it out! http://www.expresspcb.com/
In Conclusion
I built this entire project at a total cost of about $75.00 including the #25 chain 60 tooth sprocket, cable, project pc board, box, etc. which is much cheaper than the $300-$500 price tags I see for many commercial units. Implementing it on a real machine requires care, skill, attention to detail, and quality workmanship so it is not recommended for the novice. This is NOT intended nor recommended as a replacement or substitute for any manufacturer's genuine or authorized replacement part(s) and should not be used as such. I am publishing this page in hopes that the circuit and ideas presented here will prove useful to others, especially from a design perspective. I am not an electrical engineer, just a hack tinkerer ham radio guy so take that into account when considering this design.
Email me with any comments or corrections by clicking here.
DISCLAIMER
THE ABOVE CIRCUIT AND TECHNIQUE FOR IMPLEMENTING A ZERO SPEED SWITCH SHOULD BE DONE AT YOUR OWN RISK. THE AUTHOR, THIS WEBSITE, NOR THE WEB HOST ASSUMES ANY RESPONSIBILITY FOR THE SAFETY OR ACCURACY OF THIS INFORMATION. THERE IS NO WARRANTY EITHER EXPRESSED OR IMPLIED THAT THIS METHOD AND PROCEDURE IS SAFE AND NO LIABILITY WILL BE BORNE BY ANY PARTY INVOLVED IN PREPARING OR PUBLISHING THIS DOCUMENT. ZERO SPEED SWITCH FAILURE MAY RESULT IN DAMAGE TO THE MACHINE ON WHICH IT IS IMPLEMENTED AND OR PERSONAL INJURY MAY RESULT. THIS CIRCUIT AND SYSTEM SHOULD BE CONSIDERED EXPERIMENTAL AND IS OFFERED HERE FOR INFORMATIONAL PURPOSES ONLY.
USE AT YOUR OWN RISK!
