Surface Mount LED and Diode Tester

If you build a lot of prototype boards as I do, you know the problems of getting surface mount LEDs and diodes oriented properly for mounting. Even if you only build a few the frustrations are evident. The markings (if any) are nearly invisible. And the parts are smaller than pebbles. So I decided to solve the problem. Finally. I designed a battery operated (two AA cells) tester that puts out 4.5 volts maximum so LEDs are not ovrervoltaged in the reverse direction during testing. It also current limits so no more than 1 mA flows through the probes under any condition. That allows you to compare LED brightness. In the reverse direction no LEDs in the unit light up. If the probe tips are shorted a Red LED Lights. If the probe tips are aligned with the forward direction of the diode or LED a green LED lights up and a current of 1 mA through the probes will show the LED color if you are testing one.

A current mirror makes the current through the green LED the same as the current through the probes. So you get a crude visual ohm meter. All in all much handier than trying to make an ohm meter do the job. Or trying to use a power supply and a resistor. The minimum voltage across the diode at 1mA current that works with the tester is about 50mV. This voltage makes the tester functional with diodes that can handle higher currents. Higher current diodes also tend to be larger and better marked.

For the project I got the binding posts from Amazon. I also built a neat little Meter Probe Jack Adapter using these banana pins and these meter safety banana connector sockets. The banana jacks/sockets that accept meter safety banana plugs are currently out of stock. I have found no other source of supply I'd even consider dealing with. The reason for the adapter was to use this surface mount test probe without having to modify the connectors. The test probe tips need to be cleaned thouroughly to be conductive. It seems like plating would help here. I'm going to try this silver plating solution from Amazon.

You will notice I have included offset current balancing resistors to reduce the offset voltages if you use an op-amp different from the LMR342.The comparator input bias currents are so low they can be ignored in this application. The input resistor is just for protection.

The board pictured is an earlier version. I have designed a board ( 2 Sep 2022 ) that uses fewer parts. When that board arrives I will test it and announce the results. The Schematic for that board is included in the downloads. I replaced a voltage regulator and associated capacitors with a diode. A MOSFET was replaced with the output of a comparator. An output resistor was eliminated.

The schematic, parts list and parts layout are available for download here. Boards are available for $16.50 for two plus $5.50 shipping in the USA. Use the Paypal link and ask for "LED and Diode Tester 20Jan 2022 1207z". Delivery can take up to four weeks depending on availability and the USPS. You can also order the parts list directly from Mouser using this link. You can edit what you buy. I like extras because losing parts is not unusual.

The board looks like this:

Powered down.

Red LED.

Green LED.

The Adapter board and Surface Mount Test Probe. The red and black safety connecters are no longer available. The Gold Plated Banana plugs still are.

Update: 7 Sept 2022 0135z

It turns out the safety connectors are again available from Amazon. Which may mean you would like the "Dual Adapter 13Feb2022 0857z" board for $6.45 for two plus $5.50 shipping.

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