BAIDU_ CLB_ fillSlot(“365525”);
label:Test circuit (13)555 timer (35)Monostable trigger (13)
The following introduces a test circuit based on monostable trigger composed of 555 timer.
According to the conductive principle of bidirectional conductive element, the trigger reaction of monostable trigger composed of 555 timer is used to drive the buzzer to sound and display the test results.
The circuit is simple and practical.
1 circuit characteristics
Single point test: one hand holds the probe, the other hand holds the alligator clip, clamp one end of the tested element with the insulating handle, and use the probe to contact the other end of the element. Single point test is more convenient than two-point test. (see schematic diagram)

Test circuit based on monostable trigger composed of 555 timer
2 circuit principle
The test circuit is mainly composed of monostable trigger composed of 555 timer and voltage divider composed of R1, tested element and R5. When switch S1 is closed, the circuit operates randomly. At this time, a series of current pulses charge C3 through R4. In about 2 seconds, the capacitor is actually charged to full voltage, providing a stable power supply for the operation of the circuit. R4 is a current limiting resistor to prevent excessive charging current from damaging the switch.

A hand-held alligator clip insulating handle clamps one end of the tested element, and a hand-held probe contacts the other end of the element. At this time, R5 and the resistance of the measured element are connected in series into an arm under the voltage divider. The upper arm consists of a fixed resistor R1. The upper and lower arms divide the 6V power supply voltage, and the voltage divided by the lower arm is connected to the left of capacitor C1. During the test, this voltage is only a small part of the power supply voltage, which is smaller than the required 1 / 3 power supply voltage and low enough for the trigger. Its state is mainly transmitted from C1 to pin 2 of IC1.

The integrated circuit timer IC1 forms a monostable trigger together with peripheral elements. Therefore, when the above low pulse is added to the trigger input terminal (pin 2) for triggering, the output terminal (pin 3) will have a high level for a certain time, and then return to its original low level state. When the output is high, the current will flow to the solid-state buzzerThe buzzer sounds for wd1. So as to judge the quality of the tested element. This time week
The phase is determined by the values of resistance R3 and capacitance C2 connected to pins 6 and 7 of IC1. According to the figure
The value on the is calculated to be approximately 0 2 seconds, to increase the timing, increase the R3 value.
3 coupling mode
The test circuit adopts AC coupling cascade mode, and the main reasons are as follows:Even if the test probe pulse is continuously applied to the element, there is only one main pulseThe trigger pulse is transmitted to IC1 through C1. If the test element is separated from the probe, the capacitanceC1 discharges rapidly through fixed resistors R1 and R2. When there is no trigger pulse, R2Make pin 2 of IC1 high, which can prevent false operation. Equivalent to probe andIf there is an open circuit between the negative lead of the power supply, the trigger conditions are not met (i.e. pin 2 of IC1 isHigh level), the buzzer will not sound.

4 test of components
The single point tester can be used to detect household appliances, including fuses and switchesTurn off the connection of bidirectional conductive elements such as electric beads and bulbs, plugs and sockets, audio wiring, etcIt can be used to test the on-off of transformer winding, diode and triodeThe basic performance of, or the judgment of capacitance. For example, when detecting a diode, its positiveThe cathode of the pole contact probe held between fingers shall be conductive. Test of triodesimilar. When testing and judging the capacitance, a small capacitance value will cause a soundThe capacitor is fully charged and will not produce sound until it is discharged again.For larger capacitance (such as 1mf), the first contact with the probe will make a sound,When you touch the probe again, it will make a sound again, but it will gradually be fully charged. At this timeTouch the probe again and it won’t sound.

The tester has the advantages of low cost, simple and practical, and can reflect the 555 timerThe application of monostable trigger and voltage divider. At the same time, alsoIt is helpful for students to understand and master this knowledge point and improve students’ learning enthusiasmAnd initiative also make sense.


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