Logic Circuit

Digital Gates

You're simulating a circuit, it requires several digital gates, but you don't have a mixed-mode simulator. What to do? One solution involves creating simplified versions of the logic functions. To do this, we look to the NMOS transistor implementation of logic gates where the transistor acts like a voltage-controlled switch. But, instead of the transistor, we'll use the SPICE switch. Just like the transistor, the switch is defined to turn ON when the input voltage goes HI...

Hi-Low Logic Digital tester

This tester, the logic level digital signals indicate on a 7-segment display. The display shows an H as the input signal is high. When the signal is low, the display shows an L on. If the input is open, the display shows N. The circuit consists of a TTL NAND IC type 74LS00 and three transistors. The NAND gates ensure that the right segments of the display can be controlled. ..

Logic level relay controller

The schematic below illustrates 4 methods of controlling a relay with a digital logic signal. Figure (A) can probably be used in most cases where the relay coil requires 100 mA or less and the input current is 2 milliamps or more. The resistor value (R) is determined from the input voltage and the available current. For example, a 5 volt input signal supplying 2 milliamps would require (5-.7)/.002 = 2150 ohms, or a 2.2K standard value. If the transistor has a minimum current gain of 50, there will be 100 mA of current available for the relay coil. ..

Logic-Level actuated gas solenoid

Despite exhaustive searching, every reasonably priced solenoid valve for controlling propane during pushbutton start required 12VDC, beyond the capabilities of a normal pump battery. I know that there are methods to generate HV pulses which will actuate the solenoid, which will remain closed down to below 5V, but I went the more expensive and reliable route with a DC-DC converter, shown here, the Newport NMR101. R2 is 10K, and R3 is 100K. R1 is 39 ohms. ..

Data Switch with LED

Just to prove that transistors can also be useful in logic situations; this is a data latch and indicator which also is a data switch and has a bidirectional input/output port which can be read or switched. If the second transistor is off, a small currrent will flow via the LED, 12K and 220K to make Tr1 conduct, keeping Tr2 off. At the same time, the 4n7 charges up. ..

Auto poweroff for power supplies circuit

Probably the easiest way of doing automatic switch off is with relay logic. In the diagram, the box marked RL1 is the coil, the 2 in the coil box tells you there are two sets of contacts somewhere on the circuit, operated by this coil. In this simple circuit, it's easy to see them!Sw1 is a momentary push switch. When it's pushed (contacts closed), voltage from In is applied to the coil so the relay operates. RL1/1 (the first set of contacts on RL1) closes and applies power to Out. RL1/2 closes at the same time and shorts out Sw1, maintaining power to the relay coil. So Sw1 is a push to switch on and the circuit is a relay bistable - bistable as it has two stable states, on and off...

CW Transmitter 5 Watt

This is a very simple 5 watt CW TX based upon a TTL logic chip. There is just one "tricky" component and this is Cx. This component should have an impedance of about 10 - 50 ohms at the frequency of interest. If you wish to reduce the transmitter power, increase the value of Cx. It is Cx which causes the square wave from the output transistor to approximate a sine waveform. The value of Cx is the price of simplicity in this TX...

Audible Logic Probe with LM339

When testing circuits with a logic probe, it is sometimes difficult to watch the LEDS on the probe to determine the logic state. With this probe the logic states are audible. This probe is designed for TTL circuits only but could be modified for CMOS. The way it works is as follows. The 5 volt power source will be the circuit under test. Clip the ground input of the probe to the ground of the circuit being tested. The other input lead is used to probe the different chips of the circuit being tested. Any input greater then 2 volts will be high and output a high tone through the speaker. Any input less then .8 volts will be low and produce a low tone through the speaker...

Logic Checker

Simplified logic checker. Logic voltage levels have the ability to display LED (Of course I (^_^;)." H "in the LED is green," L "and red LED lights, and so both the open and goes out . But not only that was put on the ability to detect narrow pulses. In doing Shottomaruchibaibureta be extended to a short, pulse, this pulse of about 100ns in LED flash you can see it. This is handy for checking the presence of the signal and strobe...

Logic Probe with PIC12F683

This project is based on a probe logic states, capable of measuring levels from TTL (5v) to state levels of PLC's (24v). For this we have employed the use of the PIC 12F683 microcontroller, which by its nature is capable of operating at low voltages, in this case 3vcc, besides having analog inputs and internal oscillator. The circuit is supplemented by an input stage which will adapt the signal to the levels between the microprocessor is able to work and another output stage that will visually show the state in which the point is measured...

Logic Probe with 74LS47

This circuit is a Logic Probe. It indicates the logic state of the node of any TTL logic circuit. To do that, we have to supply the probe with the same power of the circuit that we want to analyse: same Vcc and same GND. To check the logic level, we must connect the "Test" wire of the probe to the desired node of the circuit that we want to check. If the level is Low, the probe will display a "zero" (0) and only the green LED will be lighted. If the level is High, the probe will display a "one" (1) and only the red LED will be lighted. If the level is Impedance, the probe will display a nothing and no LED will be lighted...

High Speed Logic Astable Multivibrator (MC74HC04)

The MC74HC04 IC is a low cost CMOS Hex Inverter. I used this type mainly because, it is what I have, although you can use LS type but, with a little modification or exemptions....

Here's another daft yet interesting PIC12F675 idea I came up with. When I wanted to show someone how logic gates worked I could only find a NAND gate which wasn't very handy for demonstrating AND's OR's, NOR's and ExOR's. I also wanted to have a play with the A/D converter on the 12F675 so I came up with the idea of a PIC that could function as a single 2-input logic element. The logic function is determined by an analogue voltage applied to the GPIO4 pin when the device is first reset, it isn't sampled again after this so logic can't be changed on the fly. The 3 MSBs of the A/D conversion give eight distinct voltage levels that map to specific logic functions...

The circuit shown will convert logic level voltages to and from RS232 serial voltages levels. For basic serial comms between a Microchip PIC and a PC or other device you generally only need to connect the Gnd, Txd and Rxd lines. This little circuit will work with a supply voltage of 3.3v to 5v making it ideal for interfacing with a PIC ..

When inserting a logic device in a live circuit the most important rule is power sequencing. For instance; ground, disable, and power should be connected in this sequence through a staggered edge PCB. Another hint is to make sure the disable RC time is about 10 times less than the power RC time as shown in Figure 2. This insures that the part will be disabled and wont disrupt bus data during insertion. Actually all logic families should follow these hints...