Homemade digital multimeter displayed intercom, Field Stregth Meter
Keywords: homemade digital multimeter display intercom
Homemade digital multimeter display intercom
Author: Luo Yuping
First, the fabrication of the signal receiver is very simple (see the accompanying drawing) that the receiving circuit consists of only 4 elements. To reduce the receiving antenna size, the commonly used 450MHz ~ 470MHz band is used to design antenna. The center wavelength of the stage is 65 cm, and the antenna L is selected from the copper heart plastic line of φ2mm, length 65 cm, and processes it into a circular antenna of φ200 mm. The antenna is equally applicable to the walkie-talkie of the VHF band. The detection diode D is a key component, requiring a cutoff frequency greater than 500MHz, and the mixed diode removed from the mechanical U head on the old TV set is most suitable, its operating frequency is greater than 900MHz, its model is 1ss99, 1ss86, 1ssl6, etc.
Capacitor C uses a monolithic capacitor that stabilizes test data to make it unaffected by external interference. The light-emitting diode uses a high-brightness red luminous tube, and the bright voltage is 1.6V is best. It uses the effect of the receiving voltage when the 0 ~ 30cm is close to the test walkie-talkie emission voltage. If the light tube is not used, the 2W machine can reach 25V when the test voltage is lowered.
Further, the arrangement of the light-emitting tube can be judged or contrast the size of the emitting power based on the strength of the tube light without using a typical meter. When the light tube is slight, the measured transmit voltage is 1.6V. When the shiny tube is brighter, the measured transmit voltage is about 2V. The digital multimeter for display can use any model of large-scale meter, and their pin distance is 19mm, which can be easily plitted in the signal receiver made in the figure. According to the far close to the test distance, select the DC 200mv, 2V, and 20V gear in the digital multimeter. If you don't have a digital multimeter, use a dynamic meter or 50μA to 300μA DC header (but between the thermal zone header is typically between 600q 4kN, the received signal power is partially consumed, so that the test The sensitivity is reduced), and the internal resistance of the digital meter is more than 10mΩ, so the test signal loss is very small.
The antenna and components of the receiving circuit are welded on a 18 mmx43mm printed board, which is arranged.
Second, the use of the self-production field strength meter measured the measured data as the emission voltage value, using the same type of walkie-talkie to the emission voltage value measured in a fixed distance, the strength of their transmit power, the following list Test data for seeds (see Schedule).
model
Nominal power
4m test
20cm test
L0CM test
T4558
0.5W
180mV
890mv
1.65V light
HWL688
O. 5W
160mV
850MV
1.55V light slightly light
JT3118
2W
650MV
1.7V light
1.95V lamp bright
Note: The above-mentioned fixed distance refers to the availability of data in the multimeter table, usually in the range of 0 to 6 meters.
In addition, the field strength instrument can also be used to test the transmit voltage value of the radio emission device such as a mobile phone, a cordless telephone.
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