74xx Series Logic
August 8, 2013
Next it's time to look at logic gates. I'm using the 74LS series of chips, because I have a lot from experiments 20 years ago. The modern equivalent is probably the 74HC series as recommended in Make: Electronics: Learning Through Discovery
NAND Gates - 74xx00
The first chip we're going to play with is the 74LS00, a quad NAND gate. Here's the pinout diagram:
Power goes to pins 7 and 14, and the others provide access to 4 NAND gates. Power should be provided by a smooth 5V. The recommended way to do this is using a 7805 voltage regulator, with an capacitor of 0.47uF on the 9v power supply side, and a smaller 100nF capacitor on the 5v side.
Generating TTL Logic Signals
Use the following circuit to generate TTL Logic Signals appropriate for using as the input to all 74LS logic chips.
The resistor in this case is 1K.
Whilst the switch is open, the input is held high and gives an On signal to the logic input.
Whilst the switch is closed, the input is pulled low, and gives an Off signal to the logic input.
Detecting TTL Logic Signals
It is possible to power an LED direct from the logic output of a 74LSxx chip, but if you use this method, that's all the output will be able to drive. However, it is more useful to use the following detector circuit to drive an LED. This allows multiple receivers, such as multiple LEDs or multiple other logic gates to work together.
In this example, R1 is 27K, R2 is 270R and Q1 is a BC548 or similar.
AND Gates - 74xx08
If the chip is replaced with a 74LS08, it has the same pinouts but this time provides four AND gates.
Other logic gate chips in this series include:
- 74xx02 - 4 x 2 input NOR gate
- 74xx04 - 6 x NOT gate
- 74xx10 - 3 x 3 input NAND gate
- 74xx11 - 3 x 3 input AND gate
- 74xx20 - 2 x 4 input NAND gate
- 74xx21 - 2 x 4 input AND gate
- 74xx27 - 3 x 3 input NOR gate
- 74xx32 - 4 x 2 input OR gate
- 74xx86 - 4 x 2 input XOR gate
Latching
You can get a two input AND gate to latch its output On. Take the first input of the AND gate and permanently make it On. Take the second input, and make it switchable. In this way, when the switchable input receives an On, the AND gate receives two On signals and the output is On, which can be used to light an LED. If you connect the output to the switched input via a diode, then the On from the output is fed back into the switched input - and the gate latches On once the switch has momentarily been set to On.
