Hacking Hobby King Volt Meter

Hacking the Hobby King RC 3 Digit Volt Meter To read below 3.2 Volts
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Hobby King has a cheap (Less than $3.00) volt meter designed to be installed in RC cars, planes etc to monitor the battery voltage. [Hobby King Volt Meter] The range on these meters as they come from Hobby King is 3.2 to 30 Volts DC

These seem to be a very popular item and are often out of stock. I am in the US so I ordered 10 of them and the chipping was only 2.99, which I think is a good deal. If you place a single order with them while they are out of stock do not order anything else unless you want to wait on it all. Hobby King will hold the whole order until the out of stock item comes in.

I wanted to add these to some of the projects that I have going on as well as add them to some small adjustable power supplies I have on the bench.

Even though I rarely use my power supplies at under 3.2 volts it would be nice if I could read a lower voltage. After a little playing around and investigation with this volt meter I determined that the 3 volt regulator was dropping to 0 volts on the output once the input voltage reached about 3.2 volts. The regulator is a 7130-1 3 volt regulator. So the solution is to keep the meter and regulator powered up to read lower voltages. This does require I separate supply to keep the meter running and they share a common ground so no negative voltages can be measured, which is just fine for my application.

So I cut the input trace to disconnect the the 7130-1 3 volt regulator from the input terminals. I then added a 9 volt battery to the input (Top Tab) of the regulator to feed power the meter for testing.



Steps to duplicate :


Remove the double sided tape from the back of the meter.

Cut the trace leasing to the 7130-1 3 volt regulator on the top of the circuit board. The trace is on the lower right of the board and goes from a set of small pads and travels back around the small capacitor. I cut it with a hobby knife. Check your work with a volt meter from the + Battery input to the pads on the top of the board that lead to the input of the voltage regulator. Make sure you cut all the way through the trace.


Cut





Solder your new supply wires to the input of the voltage regulator This is the tab on the top of the regulator. Left most red wire in the bellow photo. Be careful when you solder the wire onto the voltage regulator. It does not take much heat to loosen the whole regulator from the board.

The ground wire can go to the ground side of the unused cap or the unused ground on the input connectors. Left most Black wire in the bellow photo.


Back



Power the board up with your external battery, or power source and the meter should read 0 volts. If it reads the voltage of the battery you did not get the trace on the front of the board cut all the way through.



Front


The meter works good and tracks well with by multimeter for anything over .5 volts. Under .5 volts and it does not always see the change in voltage, if the voltage is rising from 0 to .5 volts. It does seem to read better at under .5 volts if it is reading a decreasing voltage.

There is a small pot on the back of the meter that allows you to calibrate the voltage display if it is off. It will only swing the voltage about 1/2 of volt or so.



You Tube Video of modified meter in action.


I have not tested the input to see if it will work over 30 volts as I have not had a need to measure anything over 30 volts, and I did not want to take the chance and blow up one of my expensive $3.00 meters.


Dale


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