DIY Overheat Warning.
This simple DIY sensor can be used to warn of an abnormal rise in temperature in just about anything that can be damaged from overheating. The sensor is set to activate a buzzer and warning light when the temperature exceeds normal operating temperature. The sensor should be located near the source of the highest heat in the device being monitored. On an engine for example, it should be located as close as possible to the top centre of the block/cylinder head area as this is normally where overheating will first be sensed..
I have made a few different units so far using the vehicle's 12v or 24v battery for power which work well but for simplicity and ease of installation I use a 9 volt battery, which has a long shelf life to power the alarm, making it completely independent of the vehicle, or machine's, or tank's electrical system so there is no necessity to interfere with any electrics at all. This also allows the sensor to be used safely on mains powered systems for monitoring hot or cold systems. It is necessary on a vehicle to route the twinflex through the firewall to connect the sensor to the warning box.. Adjustment is made by moving the contact pin in the lid away from the coil enough so that normal operating temperatures do not activate the alarm, but a significant rise above normal temperature will warn timeously of this event. Ideally it should be easily accessable to enable the readjusting necessary to move the pin so that the alarm doesn't activate on an unusually hot day or under heavy load conditions that strain the cooling system but are not damaging to the engine.
Step 1: Making the Sensor
Parts needed for the sensing unit are:
Bimetal coil, suitable small tin with lid to house the bimetal coil,.screw with nut, insulating washers and terminals for the lid.
The sensor is simply a small round tin and lid of suitable size into which a small bimetal coil is mounted fixed in the centre so that heat causes the coil to close resulting in the end tab of the coil moving around it's axis near the edge of the tin till it touches the insulated contact pin located in the path of the tab. The lid must be able to rotate on the tin to allow for adjustment of the temperature at which the alarm will activate. The sensor is mounted in close proximity to the heat source with a clip, bracket, or even silicone adhesive, or any means that will allow easy access to facilitate adjustment of the position of the contact pin. Once the required positioning of the pin is set above the normal operating temperature further adjustment should not be needed. A reference line should be marked down the side of the lid and container as the normal setting point.
I have also used an adjustable bimetallic strip thermostat (KST series) from a tabletop hotplate or electric iron to sense for overheating on my car. I mounted the thermostat about 5mm away from the engine block. As the thermostat's points open on rise of temperature I had to put a small 5 pin relay in the box to convert the thermostat points opening into the relay's points closing. A simpler way to use this would be to obtain an adjustable thermostat with points that close on rise but this is not easy to obtain. A suitable container is needed to house this thermostat and I am still working on it.
I still want to cut a round plate to mount the coil and use a Bakelite, or similar, heat resistant lid to cover the coil. The contact pin could than simply be bolted through the lid making insulating washers unnecessary.
For another variation I have used a complete choke unit by cutting a plate to match the 3 hole mounting bracket and mounting a screw in the path of the coil end. Adjustment is the same as adjusting the choke on a carburettor by loosening the three mounting screws and rotating the choke unit.
Step 2: Making the Alarm
Plastic enclosure, 12 volt piezo buzzer to fit in enclosure, 12volt Red panel light, 9volt PP3 Battery, PP3 snap connector with leads
The enclosure I used measures 70x50x25 mm and is large enough to house the battery, piezo buzzer and warning light. A hole is drilled and then filed out to fit the panel light. A hole is then made to match the hole in the buzzer at the other end of the enclosure The buzzer is glued over the hole in the case with epoxy and a hole at the back is drilled for the two leads to exit. The enclosure is attached with double sided tape or screwed into place, as you wish.