Smart Tweezers DMM HackAll
> Smart Tweezers DMM Hack by openschemes
We got a DMM Smart Tweezer free when we ordered a more expensive DMM. Originally, it could only measure R, C, and diode but upon opening it up we found it had a full-featured DMM chip inside. By cutting and jumpering some traces it was possible to enable the voltage-measurement mode to enhance the capabilities of this low-cost electronic gadget.
Read the extended version at http://projects.openschemes.com/SmartTweezer
This is the DMM chip that resides in the smart tweezers. Although the tweezers only enable R/C/D measurement mode, we can see from the datasheet that this chip does a lot more!
This table in the datasheet shows the logic and analog connections for all the different modes of the IC. Since RCD mode uses the standard input VR1, we can easily enable the voltage measurement mode with a little tweaking.
In this picture you can see the interesting pins. We need to add a resistor and pullup (one side of DPDT switch) to the FC1 pin to allow us to select the voltage measurement function. And we need to modify the traces connected to the VR1 measurement input.
Here is the wiring for the DPDT switch. Pole #1 is used to short the FC1 pin to the battery + (RED). Pole #2 is used to switch around the analog connections used on the measurement terminal. When the switch is flipped to position A, the FC1 pin is in it's normal state and the probe and PTC resistor are connected - enabling the normal RCD mode. When the switch is flipped to position B, the probe is disconnected from the PTC and the FC1 line is pulled up, enabling voltage measurement mode. Easy as pie!
Here is a more detailed picture of all the circuit board edits. The blue trace (on bottom of board) and green trace (from the probe) need to be cut. The VR1 needs to be jumpered directly to the probe with the orange trace. Then, the probe/PTC lines and FC1/Battery RED lines are connected to the DPDT as shown above.
Here is the picture of the back of our gadget, showing the installation of the DPDT switch.
And here is a really funny warning - don't put your testes on a live circuit!!
You can see more details on our project page, including the data from bench test comparison between this cheap meter and an expensive Fluke DMM. Result - this thing is not too bad once it's calibrated! Oh, and we show you how to calibrate as well.