TTL Logic Gate Tester

Hobbyists use a lot of TTL logic gates as they are common and readily available. I know I use a lot of logic gates over and over in various projects, I wanted a way to test the IC's before using so I do not go astray when troubleshooting a circuit when I could have verified a gate first, before using it. Not only does this tool check the 3 basic gates, NOT, NAND, and NOR, it also checks AND, OR, XOR, and XNOR gates.

This is a simple logic gate tester to test ICs with 2-input quad gates as well as hex NOT gates. This is primarily for TTL gates (7400 series) however, CMOS gates in the same "family" can be tested, such 7408 is TTL AND gates, as well as 74LS08 is low power TTL AND gates, and 74HC08 is low power CMOS AND gates, but the CD4081, which is in the CMOS "family" (4000 series), cannot be tested in the same socket as it has a different pinout configuration.

This is not used for testing gates with open collector outputs.

This tester is built around three ZIF sockets, LEDs to indicate gate status, and two buttons to provide LL1 each when pressed. Power input is a standard jack (5.5x2.1mm) from 5 to 12 volts, a regulator is built in to supply 5 volts when connected to a 9 or 12 volt supply. A set of jumpers is used to switch the center ZIF from NOR to XNOR.

Truth tables and basic pinouts are attached. Have this sheet printed and handy when you use this tool for the truth table of the gate being tested, and use the basic pinouts - as long as the pinout is the same format, you can test that IC in the appropriate ZIF socket.

Schematic is also attached.

Supplies

Parts:

14 pin ZIF socket, x3

momentary switch, x2 (I use the 2 pin type)

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

mini slide switch, SPDT 3 pin, x2

1N5819 Schottky diode, x1

LM7805 voltage regulator, x1 (heatsink is optional)

10K ohm resistor, 1/4 watt 5%, x2

330 ohm resistor, 1/4 watt 5%, x15

10 uF 25 volt electrolytic capacitor, x2

100 nF (symbol 104) ceramic or monolithic disc capacitor, x5

LEDs, 5mm green, x15

dual row male header pins, 4 pins long, x2

header pin jumper cap, long type, x4

circuit board, x1 (I wrote the schematic and board layout and had a board made through PCBWay. Gerber zip is available for download in Step 1.)

standoffs and screws, x4 of each (to keep the board off the work surface, I used 3mm x 6mm - screw diameter by height )

DC wall adapter, 5, 9, or 12 volt, center pin positive

Tools:

soldering iron & solder

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

wire cutter

electrical tape

Step 1: Order the PCB

I have placed the gerber zip on my proton drive and you can download that from here:

https://drive.proton.me/urls/0NYGBM0SHW#59fk8DZmXGAM

Once you have the zip file, you can use that to order PCBs with a manufacturer of your choice.

Step 2: Standoffs & Smallest Parts First

First, attach the standoffs in the corners with screws. I use nylon standoffs and screws, 3mm thread size and 6 mm height. Then place and solder in the resistors and the diode. Check that the solder connections are good (reflow if needed), trim the leads. Use electrical tape or tape of your choice to hold the parts in place and just solder in as many at a time as you are comfortable with.

Resistors R1 and R2 are pulldowns for the LL1 switches SW1 and SW2.

Resistors R3 through R17 are current limiting resistors for the LED's.

D16 is to prevent 5 vdc from going back to the regulator output when powering the circuit with a 5 vdc power supply.

Step 3: Small Capacitors, Momentary Switches & LEDs Next

Now place and solder in the small capacitors (I use monolithic) and then the LEDs. Check the solder connections, trim the leads.

Capacitors C2 and C3 are filter caps when using the regulator to make the 5 vdc.

Capacitors C5, C6, and C7 are decoupling caps for the ICs being tested. Seems like overkill but good practice to use decoupling caps on ICs.

LEDs D1 through D14 are the gate output LEDs to indicate LL1 when lit.

LED D15 is the indicator for when power is applied to the tool.

Switches SW1 and SW2, each provide a LL1 when pressed, to the gate input leads. SW1 is used for testing NOT gates, as indicated.

Step 4: ZIFs, Slide Switches, Header Pins, Regulator, Caps, DC Jack Next

Now the rest of the parts go in to complete the PCB.

U1, U2, and U3 are the ZIF sockets, left, center, and right, are the test sockets, each is labeled with type of gate it can test. U2 for NOR and XNOR uses header pin jumpers to select between the gate you want to test. The notch on an IC goes up, or towards the handle of the socket, as pin 1 is top left of each socket.

P2 and P3 are the double row header pins, 4 pins are used for each gate you want to test between NOR and XNOR.

J1 is the DC jack, standard 5.5x2.1mm center pin positive, you can use 5, 9, or 12 volt supply, making sure switch SW4 is set to either 5v or 9-12v.

SW3 is the on/off switch. I am not a fan of inserting ICs into the sockets with power on, so I turn this switch off, insert an IC, and when switched on, D15 will light indication you have 5 vdc power to the tool.

SW4 is a selection switch, if you are using a 5 vdc power supply, then set to the left, or 5v, this will bypass the regulator, and if using 9-12 vdc power supply, set to the right, or 9-12v, this will now power the regulator U4.

U4 is the LM7805 5 volt regulator. Only 1 IC is tested at a time, the heatsink is optional, but as good practice, I like to add a heatsink on a regulator, that is just my standard.

C1 and C4 are filter caps for the regulator and helps, along with C2 and C3, to provide a clean and stable regulated 5 vdc.

Now you have a complete Logic Gate Tester.

Step 5: How to Use

First, remove the jumpers from the center ZIF or place each on one pin only. Determine what power supply you are going to use, I chose a 12 volt supply so the next step is set SW4 to 9-12v.

Plug in the power supply and turn SW3 to ON, D15 should light to indicate you have 5 vdc available. Now turn SW3 to OFF.

I tested NAND gates first, I inserted a 74LS00 IC into the left ZIF, U1, and locked it in place with the lever. Next, turned on the power switch, and I have 4 lit LEDs, indicating LL1 at each output of the 4 gates, which also indicates the 4 gates have LL0 on each of their inputs. So far so good.

Next is to press switch SW1 to change one of each input on each gate to LL1, which still shows a LL1 at the output of each gate. So far so good.

Next is release SW1 and press SW2 and change the other input on each gate to LL1, which still shows a LL1 at the output of each gate. So far so good.

Next is to press both switches SW1 and SW2, applying a LL1 at each input of each gate, and the outputs will change to LL0, which is the correct value according to the truth table. So this NAND gate IC tests good. If your gates did not show these results, then consider the IC bad, or you can just not use a gate or gates that indicate bad.

Step 6: NOT Gates

For this test, I am using a 5 volt USB power cable from an outlet on my workbench. So with the tool turned off, I switched SW4 to 5v, then plugged in the USB cable. I inserted the NOT gate IC, 74LS04, into the right ZIF, U3, and locked in place with the lever fully down.

Then I turned on power to the tool, all gates indicating LL1 with LL0 applied, which is correct for properly operating NOT gates. (I know this truth table is not in the sheet provided, a NOT gate is also called an Inverter, so a 0 in = a 1 out, and a 1 in = a 0 out.)

Next I pressed switch SW1, which is marked for use with NOT gates, and all gate outputs changed to LL0, this IC with NOT gates tests good.

Step 7: NOR & XNOR Gates

This will use U2, the center ZIF socket. Place the 4 jumpers in headers marked P2, under the word NOR, since I will be testing NOR gates on a 74LS02. Place the IC in the socket, notched end up towards the ZIF handle and lock in place with the lever all the way down. I am using a USB cable on my workbench so I switched SW4 to 5v.

Turn the tool on and you should get 4 LEDs lit to indicate each gate is providing a LL1 output with each of the inputs at LL0 according to the truth table.

Now press SW1 to put a LL1 on one input of each gate, the 4 LEDs should turn off to indicate a LL0 on each gate output, then release SW1 and press SW2 to put a LL1 on the other input of each gate, also resulting in all the LEDs off to LL0 at each gate output.

Now press both SW1 and SW2 at the same time, all the LEDs will be off, showing LL0 at the output of each gate. This IC would be considered good.

To check a XNOR IC, simply turn off the tool, move the jumpers to P3, under the word XNOR, insert the IC, lock in place, and test.