A Logic probe is useful tool to measure, in checking digital circuits. As a meter, which is used to measure power in electrical circuits. The Logic probe will detect a logic “1” and “0” or pulse in a digital circuit.
It has a handle. At the head of a pin, for measuring different parts of the circuits, or IC pin. At handle has a state of the LED display, as a “1” or “0” or pulse.
Before use, to connect the two wires to a power supply terminal. To power the internal circuit. They can be used both TTL and CMOS system.
I collect a many types of logic probe circuits. You can build easily it yourself. There are 6 circuits, as follows.(see below!)
1.) Simple Logic Probe using IC-4050
When you learn a digital circuit. It is a necessary tool for check logic of the digital. May flee don’t pass Logic probe for extremely amateur type our electronics emphasize economize keep before. I interested circuit comes to present be Logic probe by IC 4050 by pillar equipment be IC CMOS 4050 or CD4050 or LM4050. Be the integrated circuit gate buffer logic. I uses just one part or 1/6 only. From the circuit LED stick bright when input = logic low only.
2.) Logic Digital Tester using LM324
This is Logic Digital Tester Circuit. It uses the level input about 5V and use the integrated circuit LM324. Be Main Part electronics perform Drive All LED. Then use Current very low about 10mA by have potentiometer be formed fine the value threshold give just right with the level logic digital both of 3 the level that use by LED1(Green) be the level high and LED2(Red) Be the level LOW and The LED3(Yellow) be the level High IMR . The detail is other please see in the circuit please sir.
3.) Mini Logic probe with transistor circuit
This is a Logic probe circuit is one,That is suitable for checking the voltage levels in the TTL circuit. It receives from the circuit being tested.Shows how high or low logic level one.
The circuit when the input voltage to the probe Tip is higher than 2.1 volts (state logic “high”).Transistor Q1 will run the light emitting diodes (LED) LED1 receive direct bias will come out shining. Show the logic “high”(high). When the collector voltage of Q1 running it will fall.and Since the input entered at the end of probe is connected to one end of resistors R4, joint base / emitter of transistor Q2. and Light emitting diode LED2 will get back to bias Q2 and LED2 does not work.
If the input voltage to the late Four home changed to 0 volts (low-level Logistics c), then Q1 is not the current bias is not working.
4.)LED status TTL logic High-Low circuit
This is a cycle high or low status of the digital logic signal. Since the 5V power supply is ideal for TTL digital And the circuit to clearly display text. Is easy to see the value.
Operation of the circuit is in normal state or input into Low. And it IC1 to a High state to stimulate Q1 to work with the pressure to drop across R2. That the diodes D1,D2, D3, and connect the LED 7 Segment to display the letter “L” out. If the input is marked High, it makes IC1 return status is Low the Q1 does not work, but will flow from the input through R5, R6 entry. 7 Segment and through R3, R4, D4, D5 to 7 Segment as well. the 7 Segment display letters “H” up. and the cycle will have a resistor R7 is connected with to help reduce the current flow to LED 7 Segment. It prevents damage crash.
Note: the long time ago I build this circuit with IC TTL 74LS47, but now I can not but this one so I apply to this circuit is very simple and cheap as well.
5.) Voice logic probe by transistor BC557
The key is that the logic probe circuit, using a light emitting diode, or LED display is a state (high or low) point of the analysis, but on the other hand, it can display the state has the same voice.
The circuit is shown, it is the audio source, click the tiny speaker with a repetition rate (fast or slow) Is Low status and low costs, high.
The two-transistor circuit (No. BC548 and BC557), the Multi vibrator is a simple astable. One with a 100 kohm resistor and 10 uF capacitor determines the repetition rate, the speaker serves as a load transistor BC557, is in the collection of it Peter, the negative voltage.
BC547 transistor and a resistance of 10 kilo ohms in parallel with a resistor 100 kilo ohms, the Four home inspection when conditions appear to have Carlo Magic high state (high) BC547 will work, the repetition rate increases,.
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