Environmental humidity tester system circuit design using NE555 timer

　　With the development of science and technology, society needs monitoring equipment that can be automatically controlled. Environmental humidity tester is a device used for automatic monitoring and control of environmental humidity. This article introduces an environmental humidity tester based on counter CC4060, which has a simple circuit, convenient debugging, accurate monitoring and high precision. The humidity monitoring part uses polymer film humidity sensitive capacitor HS1100, which has complete interchangeability without calibration and can be desaturated instantly. In the relative humidity range of 0% to 100% RH, the capacitance changes from 162pF to 200pF, and its error is no more than ±2%RH. The response time is less than 5 s. Under the conditions of 55%RH, 25℃, and 10 kHz, its typical nominal capacitance is 180pF. The power supply voltage is generally 5 V, and the operating temperature is -40℃ to 100℃. When HS1101 is used as a humidity sensor, there are two design schemes for the measurement circuit, one is a linear voltage output type with a positive proportional coefficient; the other is a linear frequency output type with a negative proportional coefficient. This design uses a frequency output type.

　　The humidity frequency conversion circuit uses NE555 timer, which is low cost and reliable in performance. It only needs to connect a few resistors and capacitors to realize pulse generation and conversion circuits such as multivibrator, monostable trigger and Schmitt trigger. This circuit forms a multivibrator with humidity-sensitive capacitor HS1100 and resistors. By properly setting the resistance value in the circuit, it outputs a square wave to realize the conversion of humidity monitoring quantity to frequency signal. By the high and low frequency signal, we can know whether the environmental humidity is normal. The reference frequency oscillator and frequency voltage converter both use 14-bit serial counter CC4060, which uses CNOS manufacturing process and standard DIP-8 package 14-bit binary serial counter/divider integrated circuit. The structure of the oscillator can be RC or crystal oscillator circuit. When the reset terminal of CC4060 is high, the counter is cleared and the oscillator is invalid; when the reset terminal is low, the external oscillation timing element controls the generation of a certain frequency signal, and can output 4-division to 10-division, 12-division to 14-division pulse signals. The reference frequency oscillator of this circuit is composed of CC4060 and its timing element. The generated frequency signal is sent to the D flip-flop after 12-frequency division to provide clock pulses for the D flip-flop. The frequency voltage converter uses the frequency division function of CC4060 to send the frequency signal output by the NE555 timer to CC4060, which is output to the input end of the D flip-flop after 12-frequency division. The corresponding level is generated according to whether the environment is humid, and the D flip-flop is driven to work and output the control level. The output control circuit can adopt the corresponding circuit according to actual needs. The output control part of this circuit is realized by a transistor controlling a relay. The high level output by the D flip-flop turns on the transistor to drive the relay to operate, generate an alarm signal or drive the drying circuit to work, so that the environmental humidity returns to the normal range.

　　Humidity monitoring and humidity frequency conversion circuit

　　C is a humidity-sensitive capacitor HS1100, whose capacity will change with the change of environmental humidity, causing the charge and discharge time constants of pins ② and ⑥ to change, changing the frequency of the output signal of pin ③, realizing the conversion of environmental humidity changes into frequency changes, and converting non-electric quantity into electric quantity. The resistance value of the external resistor R3 of pin ⑤ is 910 kΩ, which is very different from the 5 kΩ resistance connected inside the integrated circuit, so the general reference voltage can be considered as the power supply voltage VCC, the resistance value of R1 is 50 kΩ, the humidity-sensitive capacitor is 180pF under normal conditions, and the resistance value of R2 is generally about 576 kΩ. The potentiometer can be connected in series according to the needs of debugging to achieve the best control accuracy. From the above values, it can be calculated that the pulse period T=(R1+2R2)Cln2 output by pin ③ under normal conditions is about 0.15ms, and the frequency is about 6 667 Hz. When the environmental humidity increases to 90%RH, the frequency will decrease to about 6 186 Hz, causing the subsequent circuit to act, realizing dryness enhancement and alarm.

　　Reference frequency oscillator

　　The reference frequency generating circuit is mainly realized by the 14-bit serial counter CC4060. The reference frequency timing element is connected to the ⑨⑩ pins of CC4060, and the generated signal is sent to CC4060 from the pins. In this circuit, C1 is 0.01ΩF, R4 is 2.7 kΩ, and RP1 is a 4.7 kΩ potentiometer. By adjusting the potentiometer, a signal with a period of 0.059 4 ms to 0.162 8 ms and a frequency of 16.8 kHz to 6 kHz can be generated (f=1/2.2(RP1+R4)C1). After 12-frequency division, this signal can obtain a frequency of 4 Hz to 1.5 Hz, which is output from pin ① and enters pin ③ of the D flip-flop CD4013 to provide a clock pulse for frequency level conversion. The two transistors Q1 and Q2 form a line-and circuit. When working normally, one of Q1 or Q2 is turned on, the reset pin is low, and the counter works normally. When the 4-frequency output of pin 7 and the 12-frequency output of pin 1 are both high, Q1 and Q2 output high at the same time, the counter is reset, and the counting starts again. This circuit mainly ensures that the monitoring circuit automatically resets once after working for a period of time (0.33ms~1ms) to avoid long-term data errors affecting the normal operation of the circuit. R5 and R6 are generally 10 kΩ, R7 is 47 kΩ, D1 is 1N4148, and Q1 and Q2 are 1015.

　　Frequency to Voltage Conversion Circuit

　　The frequency-voltage conversion circuit is mainly composed of a 14-bit serial counter CC4060 and a quad D flip-flop CD4013. The frequency signal sent from pin ③ of NE555 is sent to the counter from the pin of CC4060 U2 and is output from pin ① after being divided by twelve. The normal frequency is 1.6 Hz. When the humidity increases to 90% RH, the frequency drops to 1.5 Hz and is sent to pin ⑤ of the D flip-flop CD4013. At the same time, a high level is output to turn on Q3, latch the incoming signal level, and prevent the subsequent pulse signal from entering CC4060 U2 again to prevent interference. Under the control of the clock pulse (frequency of 4 Hz to 1.5 Hz) of CC4060 U1, the D flip-flop outputs a high level at pin ① of CD4013 to control the relay to work and drive the alarm or boost circuit to work. Whether the D flip-flop works or not obviously depends on the clock pulse sent by CC4060 U1. The lower limit frequency of the pulse output by U1 and U2 is the same, which obviously cannot control the normal operation of the D flip-flop. This requires us to slightly adjust RP1 during the debugging process to make the clock frequency output by CC4060 U1 slightly higher than 1.5 Hz, but lower than 1.6 Hz (debugging by blowing air to the humidity-sensitive capacitor to increase the humidity), so that the D flip-flop does not work in the normal state of the circuit. When the humidity exceeds 90%RH, the D flip-flop outputs a high level to drive the subsequent circuit to work. R9 is 2.2 kΩ, R10 is 10 kΩ, R11 can be combined with R7 into one resistor, and Q3 is 1815.

　　Output control circuit

　　The output control circuit is realized by transistor driving relay, and the level signal outputted by pin ① of CD4013 is sent to the base of NPN transistor 1815. In normal state, CD4013① outputs low level, the transistor is cut off, and the relay is released; when the humidity exceeds the specified amount, CD4013 outputs high level, the transistor is turned on, the relay is closed, and the alarm and dryness enhancement circuit work. The environmental humidity tester composed of humidity-sensitive capacitor HS1100, 14-bit serial counter CC4060, D flip-flop CD4013, etc. has the advantages of simple operation, convenient debugging, small size, high precision, etc., and can be easily debugged successfully by general electronic enthusiasts.