555 & 741 Tester

The 555 timer and 741 op-amp are favorites of the hobbyist, and with this tester, you can check to make sure either of these components are operational before using in a circuit you are building. For the 741, you can also test 741 substitution ICs with the same pin layout. As a bonus, this tester will also provide a split rail voltage (a positive and a negative voltage required by the 741) that you can tap off and use to power 1 to several 741 op-amps in a circuit you are building.

This tool will place the IC, either 555 of 741, into an astable multivibrator circuit to test the IC, the 555 will have one LED that flashes, the 741 will have two LEDs that flash to indicate they are functioning. Only 1 IC is tested at a time, 555 in the upper position of the ZIF socket and 741 in the lower position, pin 1 on each IC is indicated on the PCB.

First thing you need to do is download the gerber zip and send to a PCB manufacturer of your choice, I have placed the gerber zip file here.

You can also build this on a breadboard or two, which I have done to show proof of concept.

Once you have the file, you can send to a PCB manufacturer of your choice for fabrication, sit back and wait for your PCBs to arrive.

While waiting for your PCBs, you need the following parts:

All resistors are ¼ watt, 5%.

custom PCB, x1 (Note: the PCB in the photos here show C2 above the ZIF socket, disregard that item as it is not used, the current schematic and gerber files for the PCB layout are correct. Later on you will see a component in the C2 area, that is a 0 ohm resistor.)

LM741, x1

8 pin socket, x1

16 pin ZIF socket, x1

1K resistor, x4

10K resistor, x4

33K resistor, x1

100K resistor, x3

4.7uF 50v electrolytic capacitor, x1

10uF 25v electrolytic capacitor, x1

LEDs, 5mm; green x1; blue x2, orange x1

DC power jack, 5.5x2.1mm center pin positive, x1

mini slide switch, SPDT 3 pin, x2

momentary switch, 2 pin type, x2

header pins, single row, 3 pins long (1x3P 2.54mm)

standoffs and screws, x4 (I used 3x6mm, diameter by height)

DC wall adaptor, 9 or 12 volts, center pin positive, x1

good LM741 x1 and 555 x1 for testing.

Tools:

soldering iron & solder

solder wick and liquid flux (for when mistakes are made)

wire cutter

electrical tape (for holding parts down when soldering)

You will need the schematic to assemble the PCB so go ahead and print that out now.

Note: Looking at my PCB here, you will notice C2 above the ZIF socket, I have removed that part from the schematic and PCB files, please disregard that item on my PCB photos shown here.

Start placing and soldering the shortest components first, which are the resistors.

Next, add the momentary buttons, working your way up to the 8 pin socket, header pins, momentary switches, LEDs, and capacitors, and finally the DC jack and the ZIF socket.

Place a known good 741 in the 8 pin socket, this IC is used to create the + and - voltages for the op-amps.

The tester has an on/off switch as well as a 555/741 switch, which powers either the 555 or a 741 (or its compatible substitution).

Test 555. With the SW1 (marked ON/OFF) in the OFF position, plug in a 9 or 12 volt DC wall plug to the PCB jack, insert a 555 into U2 test socket, in the upper position where it says 555, and place pin 1 of the IC where it says 1 for the upper position of the test socket. Push the lever forward and down to lock the IC in place. Position SW2 (marked 555/741) to the 555 position.

Position SW1 to ON position, D1 will light to show tester is on. Now press SW3 (marked Test 555) and D2 should flash on and off, this will indicate the 555 is working correctly and able to work as an astable multivibrator. If D2 does not light or stays on continuous, the 555 is considered bad.

Test 741. Now turn the tester off. Open the lever on the test socket and remove the 555 and now add a 741 op amp to the lower portion of the test socket, as indicated on the PCB, and the board shows you where pin 1 goes as well. Close the lever on the test socket. Position SW2 (marked 555/741) to the 741 position.

Turn on the tester, D1 will light to show the tester is on. Now press SW4 (marked Test 741), D3 and D4 should flash alternately between blue and orange, this will indicate the 741 is working correctly and able to work as an astable multivibrator. If D3 and or D4 does not light or stays on continuous, the 741 is considered bad.

Split Rail Power. The three header pins on the PCB can be used to power 1-2 op amps in a circuit you may build or tinker with. The pins are marked as +V/2, 0 Ref, -V/2. When you have a 12 volt supply plugged into the tester, SW2 in the 741 position, and you have to press the Test 741 pin +V/2 is the supply voltage divided by 2 (I show +6.19 volts), and -V/2 is the supply voltage divided by 2 (I show -6.26 volts). The reference for these two voltages is the 0 Ref pin, that is what you connect your ground line on your meter to and to your circuit where stated. I will work on an update to make this a switch for the voltages instead of momentary for testing the IC.