How does a multimeter measure resistance? A lot of online answers suggest that a constant current stimulus is used. I looked into it and found that the stimulus current is not constant at all for digital multimeter. Perhaps it was true back in the analog meter days. The digital multimeter uses a constant voltage with a resistor in series. For different measurement range, different resistor value is used, large resistor value for large measurement range. Some meters also change the constant voltage for different ranges, normally large voltage at lower resistance range. The series resistor and the resistor under measurement form a resistor divider. The resistance measurement is calculated from the divider voltage.
It is fairly easy to find out the stimulus voltage. First set the ohm meter to manual range. Use a volt meter to measure the voltages of the ohm meter for two known resistance. From the two equations, we can solve the stimulus voltage and the series resistance.
For Fluke 87, the stimulus voltage is 1.235V, the series resistances over the ranges are 6.5K (400-Ohm), 15.7K (4K-Ohm), 106K (40K-Ohm), 1.02M (400K-Ohm), 10M (4M-Ohm) and 10M (40M-Ohm).
For Cen-Tech multimeter, the stimulus is 3V for 200-Ohm range and 0.3V for others. The series resistance is 1.59K (200-Ohm), 3.34K (2K-Ohm), 12.5K (20K-Ohm), 103K (200K-Ohm), and 1M (2M-Ohm).
For high-end digital multimeters, like Keysight 34401A (6.5-digit), it uses current source, 1mA (100 and 1K-Ohm), 100uA (10K), 10uA (100K), 5uA (1M), 0.5uA (10M), 0.5uA||10MOhm (100M).
Tuesday, January 22, 2019
Friday, January 18, 2019
Aukey Dash Cam GPS Antenna
The GPS antenna ($20) has 4-conductor 3.5mm plug that goes into the Aukey Dash Cam for location information to be included in the recorded video.
We'll try to decipher to the interface signals. From a picture of the disassembled GPS antenna, we know the pinouts for the plug, VCC, GND, TX, RX. It is more than just an antenna; it is a GPS module. We'll take a look what the protocol is. First we want to know the VCC voltage. VCC is measured to be 3.3V, so are TX and RX. The we'll look at the TX and RX to see what kind of signals they are. There is no activity on RX. TX shows what appears to be 3.3V UART signals. The minimum pulse width is about 104us, so the baud rate is 9600. The messages come in 580ms burst every second. We connect TX to a UART receiver. We can see the standard NMEA GPS strings. The messages include GPRMC (recommended minimum), GPVTG (vector track over ground), GPGGA (fix information), GPGSA (satellite data), GPGSV (detailed satellite data), GPGLL (lat/lon data).
We can tap into the TX signal to extract GPS data for other purpose.
We'll try to decipher to the interface signals. From a picture of the disassembled GPS antenna, we know the pinouts for the plug, VCC, GND, TX, RX. It is more than just an antenna; it is a GPS module. We'll take a look what the protocol is. First we want to know the VCC voltage. VCC is measured to be 3.3V, so are TX and RX. The we'll look at the TX and RX to see what kind of signals they are. There is no activity on RX. TX shows what appears to be 3.3V UART signals. The minimum pulse width is about 104us, so the baud rate is 9600. The messages come in 580ms burst every second. We connect TX to a UART receiver. We can see the standard NMEA GPS strings. The messages include GPRMC (recommended minimum), GPVTG (vector track over ground), GPGGA (fix information), GPGSA (satellite data), GPGSV (detailed satellite data), GPGLL (lat/lon data).
We can tap into the TX signal to extract GPS data for other purpose.
Subscribe to:
Posts (Atom)