Portsdown DMM Display and Logger

The Portsdown Digital Multimeter display and logger works with a modified Parkside PDM300 C3 multimeter (seasonally available for £11.99 from Lidl) to display and log the meter readings.

Initial proof-of-concept functionality is limited to a large display of the voltage reading. Based on user feedback, further features will be added.

Modification of the DMM

Warning - this modification relies on careful construction and an opto-isolator to keep up to 300V AC away from yourself and your Portsdown Raspberry Pi. DO NOT attempt this modification if you are not confident in your construction abilities.

The DMM has an internal (live at the measurement voltage) test point where an opto-isolator can be connected to pick up a 2400 baud serial data stream. Most published modifications (https://www.mikrocontroller.net/articles/Multimeter_PDM-300-C2_Analyse#Testpunkte and https://github.com/benedikts-workshop/ParksideView) recommend fitting the opto-isolator between the test point and ground. However, this draws current for 90% of the time and significantly reduces the internal 9v battery life.

It is recommended that the opto-isolator is fitted between the test point and Vdd with a 1K5 resistor in series. This means that there is only a marginal increase in supply current. I connected the output of the opto-isolator to a 3.5mm jack socket mounted on the front of the meter.

The complete circuit of the modification and Portsdown interface is shown below.

Other opto-isolators can be used - mine was sourced from the junk box.

The amplifier transistor (BC108 or any medium-high gain npn) was required because GPIO pin 3 on the Raspberry Pi is intended for use with i2c, and I was unable to disable the internal pullup. This results in the input signal to the GPIO pin being idle = mark = 1 = low, and start bit = space = data 0 = +3.3v. The serial data is 2400 baud.

A battery capacity testing application is being developed that will monitor (and plot) the voltage of a rechargeable battery and disconnect the load when it reaches a preset "discharged" voltage. The relay for disconnecting the load will be operated from the Raspberry Pi pin 26 (GPIO 7). This is the same pin as is used for switching the Noise Source in the NF Meter, and is found on pin 7 of the Portsdown 25-way interface connector.

Use with Portsdown

After building the interface, connect the Portsdown and turn it on. Go to Menu 2, Test Equipment, DMM Display.

Then select DC Voltage on the meter; the Portsdown should display the same digits as the DMM. There is occasionally an error of one in the LSB - this seems to be a rounding error inside the DMM.

In the intitial proof of concept, ONLY DC Voltage has been tested. The ONLY buttons that work on the Portsdown are "Exit to Portsdown". There is a lot more work to do, but I wanted to release something so that other users could test their interface, think about what features they want and let me know.

Note that the screen image is also available on the web interface, but no control is possible - that must be done from the touchscreen.