Design and implementation of passive LCD module

The passive LCD module only needs to be connected in series to the 4~20mA loop of the two-wire transmitter to display the measured engineering value or measurement percentage without the need for batteries or power supply. It can be embedded into the meter head of the field instrument for local display, or installed in the control room for centralized display. It is a universal passive LCD module.

working principle

The display module is connected in series to the 4-20mA loop of the two-wire transmitter through two terminals, and a 2.5V voltage drop is generated through the module's adjustable precision parallel voltage regulator tube, and then the 5V power supply of the module is provided through voltage doubling. The maximum 200mV signal generated by the signal sampling resistor is sent to the differential input end of the analog-to-digital converter and converted into a digital quantity for display. The analog-to-digital converter uses the dual-integration analog-to-digital converter ICL7106, which is highly resistant to power frequency interference and can directly drive a 3.5-digit LCD display, as shown in Figure 1.

circuit design

The designed circuit is shown in Figure 2. The display module is connected in series to the 4~20mA signal loop through the JLOOP terminal. Z1 in the picture is an adjustable precision parallel voltage regulator with a current absorption capacity of 1~100mA. The output voltage can be set to any value between 2.5 and 30V using two external resistors R3 and R4. Taking into account the voltage drop of the module and the operating voltage of the ICL7106, 2.5V is taken here. R10, R11, and R12 are connected in series and parallel as the sampling resistor of the loop signal, which can facilitate the adjustment of the signal fullness for different display ranges.

ICL7660 is a high-efficiency, low-power low-voltage DC power converter made by Intersil using CMOS technology. It can not only convert a single power supply into a dual power supply with symmetrical output, but also achieve voltage doubling and multi-voltage output. The power supply of ICL7106 is Obtained from this. ICL7106 is also a dual-integration analog-to-digital converter produced by Intersil specifically for driving LCD. It consumes very low power and can be powered by a battery. There is a clock circuit inside the chip, and external resistor-capacitor components can be connected to form a two-stage inverting RC oscillator (R13, C1 in the figure). There is a reference voltage generating circuit inside, and there is a typical reference voltage of 2.8V between V+ and COM. source, but considering its temperature stability, an external precision reference LM385-1.2V is used here, and then the voltage is divided by a voltage divider and sent to the differential reference input end of ICL7106. The Ws potentiometer is used to fine-tune the display full scale to ensure that Vref+- Vref-=Vm/2, and Vm is the full scale. The differential input characteristics of ICL7106 allow IN+ and IN- to accept differential voltages within the common-mode voltage range. In the range of V-+1 ~ V+-0.5, the A/D converter has a common-mode rejection ratio of 86dB. In the figure, the partial pressure of Wz, R6, and R7 is connected to the measurement input IN+ terminal, and Wz is used to adjust the lower limit of the range. R8 and C5 form a low-pass filter to filter out interference. M1 is used to drive the decimal point, and DPS is used to select the decimal point display position. LCD SI71263 is a small three-and-a-half-digit LCD monitor produced by Changsha Sunren Company. The entire circuit is simple and practical. It uses surface mount packaged components to make a small liquid crystal display module, which can be directly embedded into the header of the field instrument.

The implementation method of the passive liquid crystal display module is feasible, has the advantages of simple design, reliable operation, etc., and has good reference value and practicality.