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What is a TinyOptoProbe

A TinyOptoProbe is a logic probe with an opto-coupled front end. It is useful for probing circuits with floating grounds or cards which do not have accessible grounds. It can also be used as a higher voltage DC logic probe.


Admittedly this is a very strange board. It's not easy to describe where it would be used. In 40 years of working with electronic equipment I have only needed this one time. But, when I needed it, there was no other alternative.

I needed it to debug opto-isolator inputs. I couldn't get to the output that was driving the opto-isolator because it was an a conformal coated card. Add to that the far end was driving the opto-isolator low with an open collector driver. The other side of the source was connected to a resistor pulled up to 5V power. There was no place I could hook a scope since there was no externally accessible ground point. I could probably have attached an LED in place of the opto and watched the light go on and off but what if there was a short pulse which I could not see? I needed an old fashioned logic probe but with a major difference. The logic probe had to be completely isolated from the unit under test and old fashioned logic probes require attaching the the ground. So I came up with the TinyOptoProbe.


This board allows signals to be monitored in an isolated manner from the circuit being monitored. The downside is that it requires 10 mA from the circuit under test to operate. This board is used where measurement of an opto circuit doesn't allow ground clips to be connected to the device under test.


  • Isolated Opto-coupled input
    • 10 mA nominal input current loop
    • All current is returned on the test ground
    • No current loop from the Unit Under Test to the logic on the card
    • Isolation voltage dependent upon the opto-coupler selected but typically 500 V min
  • 5V, 12V, or 24V input levels
    • Works with relay circuits for monitoring relay contacts
  • Two LEDs (High/Low and Pulse)
  • ATTiny85 microproceasor based
    • Program to monitor the input and flash the Pulse LED if there is a change (H>L or L>H)
  • DC power input
    • 9V power
      • Can be run from a wall wart power supply or a 9V battery
    • Low power consumption, 25 mA max
    • 25 mA when both LEDs are on
    • 10 mA when one LED is on
  • Small form factor 26x49mm
  • (4) 4-40 mounting holes

Operation - Test Opto-Coupler Connection

Disconnect the opto-coupler from the circuit under test Connect power to the DC power jack When the circuit under test is active the HIGH LED will light When there is a change of state the PULSE LED will light

Input Circuit





H1 VREG power header (X1 board only)

  • Install shunt to power card from the DC jack.
  • Remove shunt to program Microprocessor from ISP connector (H2).

H2 ISP Connector (X2 Board)


This is a strange shaped 7-pin header. This header serves a dual purpose.

  • Pin 2 is used for both the ICSP and power select jumpers.
    • This effectively prevents the board from being powered by both the ICSP and voltage regulator at the same time.
    • If a shunt is installed into pins 2 and 7 the power for the card will come from the voltage regulator on the card.
  • Pins 1-6 are the normal pinout for In-Circuit System Programmer (ICSP) pins.

H2 ISP Connector (X1 Board)


J1 - Power Connector

  1. GND - shell
  2. +7 to +9V - center pin

P1 - Input Connector

  1. GND
  2. +5V
  3. +12V
  4. +24V


X2 Improvements over X1

Combined power select jumper and ISP jumper into one part. This prevents the ISP cable and power jumper from being installed at the same time. It is a strange shaped part, but saves board area as well.

Checkout of Rev X1 CCA


There were a couple significant problems with the X1 circuit card.

  • Transistor Q1 had base and emitter swapped
    • Resulted in lack of input signal
    • Rework via flipping part over, bending leads down and soldering the part
    • Result of rework - input problem fixed
  • Must have made a change to the routing of one of the LED lines without running DRC at the end
    • Resulted in a power-ground short
    • Rework
      • Drill out via
      • Cut etch
      • Add wire

Kicad Schematic SOT23 pinout


Kicad Layout SOT23 pinout


Transistor Datasheet pinout


Via drillout, Etch cut and wire add


X0 Version of the board

  • Built on the TinyGrid85.