# Humidity Detector Circuit

This humidity sensor consists of 2 copper conductors that are located at small distance from each other. The relay switches as soon as the moisture makes a connection that is more or less electrically conductive between the 2 electrodes.

How does the humidity detector works?

When the electrical resistance between the 2 sensors drops below a certain value then the Schmitt trigger (T1 and T2) switches. The RS N1/N2 bistable multivibrator is flipped through C1 so that in point B we have a low voltage and so T3 will close the relay.

The relay is opened when the 10K resistor is connected to point A, not to point B as shown in the schematic. You can use other sensors like LDR or NTC instead of the copper ones so you can use this circuit for detecting light or temperature.

## Related Tutorials

• MrWhite

Colin,

Would appreciate it if you would tone down to keep this discussion educational.

http://i1147.photobucket.com/albums/o543/teehowe/circuit1_zps328062a0.jpg

Anyway, some simulations. Keep an eye on the circled nodes, Vsensor is a simulated sensor. Typically the sensor voltage won’t reach the full rail voltage; it depends on the ratio of voltage divider. In cases of NTCs and PTCs, the resistances are exponential at extreme ends of the temperatures.

http://i1147.photobucket.com/albums/o543/teehowe/Circuit1_plot_revb_zps162ef927.png

V_T1_C is the collector output of the T1 NPN while V_ff_in is the SR flipflop input buffer.
Note on the simulation, nothing actually happens at the other end of the capacitor. It is pulled up high regardless of the schmitt trigger output. The only activity we see is a positive spike and a negative spike (which is due to the change of voltage at the schmitt trigger output; I=Cdv/dt). Get a book on electromagnetics to get a better understanding of capacitors.

Another thing is that the “Schmitt Trigger” is hardly a schmitt trigger as there is no hysteresis. The condition for Vout to be high or low happens to trigger at the same voltage of ~740mV. If you want to make very reliable Schmitt Trigger, use a comparator with a resistive feedback to provide hysteresis.

I will come up with a discrete version of schmitt trigger with analysis soon. Be right back.

• Colin Mitchell

“What does not work is the output of the Schmitt Trigger will not provide any useful input to the next stage (flip flop).”

Just build the circuit and stop showing everyone you don’t have an understanding of electronics.

• Mr White

I beg to differ. The schmitt trigger works fine on its own with some tweaking.

What does not work is the output of the Schmitt Trigger will not provide any useful input to the next stage (flip flop).
Just imagine an open circuit between the Schmitt Trigger output and the flipflop input.

• Colin Mitchell

“The capacitor prevents any constant voltage (DC) to be transferred to the following stage. The only signal that can pass through the capacitor is a transient signal or a varying one (AC)”

You don’t understand the concept of a Schmitt Trigger.

• Mr White

I rephrase my intentions:

The sensor is a block containing resistive components (typically 1 resistor and 1 variable such as a sensor etc. will provide a voltage reference into the base of the NPN.

Whether the resistance goes infinitely low or infitinitely high, nothing actually at the input 1 of N1. The capacitor prevents any constant voltage (DC) to be transferred to the following stage. The only signal that can pass through the capacitor is a transient signal or a varying one (AC).

• Colin Mitchell

The sensor supplies DC input.
The sensor does not “supply DC input.”
The sensor is a resistance that changes resistance as the humidity changes in moisture-content.
I already explained before that the Schmitt Trigger will suddenly change considerably at a certain point when the resistance is a certain value.
This causes the circuit to change states.

• Mr White

T2 collector output is connected to capacitor C1.
Nothing will actually happen.
The sensor supplies DC input.
DC signals are blocked prior to the flipflop.

• Colin Mitchell

The 100p is “pulling against” a 4k7. A bad design.

The operation of the circuit needs explanation.
The first two transistors form a Schmitt Trigger.
Diode D1 serves NO PURPOSE and can be removed.
The designer of the circuit states that as the resistance between the two sensors decreases the circuit will latch the relay. This is NOT TRUE.
Let’s look at it:
The first transistor is NOT TURNED ON. (It is the same as being removed from the circuit).
The second transistor is TURNED ON. The voltage on the collector is LOW and the 100p is charged.
If the sensors are connected together, the two transistors will change state and the collector of the second transistor will go HIGH but this will not change the state of the latch.
The sensor has to go from a low-resistance state to a high-resistance state for the circuit to change states.
When the sensor is low-resistance, the maximum collector current is flowing through the 2k7 and a small voltage is developed across the 56R.
As the sensor-resistance increases, the voltage on the base decreases slightly and T1 turns OFF a small amount. This turns ON T2 and the current through the 39k adds to the current in the 56R to produce a slightly higher voltage across this resistor. This reduces the base-emitter voltage for T1 and has the effect of turning of T1 slightly without and change in the sensor resistance.
In other words the circuit suddenly changes state without any change in the input conditions.
This pulls the left lead of the 100p down and triggers the latch to changes states.

• Colin Mitchell

“Salt dissolves easily in COLD water. Much more difficult in hot water.”

This is entirely INCORRECT.

NaCl (common salt) dissolves more-easily in hot water and slightly more salt dissolves as the temperature increases.

Get your facts correct before espousing this nonsense.

• Clive Grant

There is a fundamental flaw in the preparation of your saturated salt water solution. Salt dissolves easily in COLD water. Much more difficult in hot water. Schoolboy chemistry. Think of the oceans!

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