Electronic Measureing Device
- Alison and Dave Roberts
- May 22
- 4 min read
Updated: May 26
Aug 15th 2022
When I started turning cowboy hats, I was using a light bulb in a jamb chuck to gage the thickness of the top of the hat. I first used an incandescent bulb but it would burn out while turning due to vibration. It was difficult to remove the hat, replace the bulb and then return the hat to the jamb chuck perfectly as it was. The tolerance is very fine so any wobble is not acceptable. I then decided to us a modern LED bulb, this presented a new challenge as the wattage is not close to the incandescent, I was using so gaging the thickness was still difficult. As I have some knowledge in electronics from my working career with CN Railway, I decided to build a simple electronic device to measure thickness. This device will measure up to almost 1/2" thick material but is better suited to 1/4" and less. It is very simple, inexpensive and useful to gage thickness wherever you can’t reach with a caliper or fingers. Once you turn enough hats you can gage the thickness by the sound of your cut but this takes many hats and a keen ear, this device helps reassure your thickness.
The above photo is a cowboy hat on the jamb chuck with a light bulb inside along with a rare earth sphere magnet. The hall effect transistor is mounted to a thin piece of wood with 2 rod style 1/4" rare earth magnets which are drilled into the end of the wood and sits just proud. Once the magnet on the wood device finds and attracts the sphere magnet inside the hat the transistor conducts and outputs a voltage. The transistor output is fed to the digital multimeter and provides an accurate representation of the thickness. Simply slide the device around and the sphere magnet will follow, when the material changes in thickness that will be represented on the multimeter. This device needs a little bit of fine tuning to place the rod magnet in the wood with respect to where the transistor gets mounted but is straight forward. The meter voltage is simply a pre-determined value by measuring a piece of flat wood say 2 mm thick, the meter should read about 4.5 volts. Then when your work piece is 2 mm thick the voltage reading should be the same at 4.5 volts DC. As the wood gets thinner that voltage will increase as the transistor is getting closer to full saturation. The supply voltage is 5vdc and if you removed the wood and placed the two magnets together then the transistor will saturate and your meter should read full supply voltage.
I used an Apple iPhone wall adapter with a USB port and then took an old pc mouse with a USB cord, cut the cord off the mouse and that became my 5-volt supply. I purchased a Hall Effect transistor in a little package; it came mounted on a circuit board with a resistor and capacitor along with the connector and cable for $12. Using a digital volt meter for the display and lastly a 1/2" sphere magnet with two 1/4" rod magnets and some wood. The circuit is simple and can be made by anyone that knows how to solder and use a voltmeter. I used 3 pieces of arbutus (Madrone) to match the density of the wood to the wood I am turning. The desired thickness is 2 mm the other pieces are 4 mm and 8 mm so as to get a reference to thickness with the meter. It took some minor tweaking with the magnets mounting them in a thin piece of wood and then mounting the circuit board so as to get a voltage output on the meter. Basically, the circuit conducts with the presence of magnetism and then outputs a voltage. When I place the sphere magnet onto the probe magnet the transistor saturates and outputs the nominal 5-volt supply voltage. With a 2.5 mm piece of wood between the two magnets the transistor outputs 4.5 v dc and with the 4 mm piece of wood that changes to about 4.2 volts. So, if you’re turning any object that you need to measure thin wall thickness you can use this device. As an example, my calipers don't fit inside a vase or in this case the hat so I put the sphere magnet inside the jamb chuck and use the probe on the outside so it attracts to the sphere magnet and see what the voltage on the meter displays. The lower the voltage the thicker the wall thickness - the hat in the above photo reads 4.64 vdc so the thickness is less than 2 mm.
Parts required.
Magnetic sensorwww.Phidjets.com P/N 1108
Hammond Box 1591-A
Hammond Box 1591-M
1 red and 1 black female jack that accepts your meter leads.
2 each 1/4" by 1/4" rare earth rod magnets
1 each 1/2" rare earth sphere magnet
Your ingenuity in swapping incandescent for LED bulbs to gauge hat thickness demonstrates creative electronics skills. Building a simple thickness-measuring device that handles half-inch material with precision reduces downtime and improves accuracy. Such a tool is invaluable when vibration disrupts bulb-based jigs, ensuring consistent wall thickness without repeated calibration. Similarly, for display diagnostics, load a blank black screen test to uncover backlight uniformity, reveal pixel response, and validate panel calibration under true-dark conditions. Check it out at black screen onl for a seamless Full-Screen Charcoal View experience that streamlines display testing, conserves power, and reduces eye strain across TVs, laptops, and smartphones.