Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Humidity sensor problem

Not open for further replies.


Advanced Member level 4
Jun 11, 2004
Reaction score
Trophy points
Activity points

I am going to use Humidity sensor HS1101 from Humeril.
I studied the dtasheet but its very confusing and i am unable to understand it properly

Have any body ever used this sensor?
I want to know how to use it mean i want the circuit diagram.

A circuit is given in datasheet which converts the capacitance change due to humidity in to frequency.Now by measuring this frequency through a mcu i can find out the relative humidity.but the formulae given in datasheet are very confusing to me
Somebody plz explain them?

Datasheet also talks about the STRAY capacitance.what is it?
also it is said to pay attention to soldering what soldering considerations should be there


Advanced Member level 1
Feb 3, 2005
Reaction score
Trophy points
Activity points
used sensor hs1100


I downloaded the datasheet and i took a look at it.
Because the sensore works as a capacitor and the capacitance changes over HUMIDITY. becuase its not easy to read capacitance, They built a simple circuit using the 555 so the output will be frequancy.
If you will look at the table of Typical Characteristics for Frequency Output Circuits you will see that the more humidity you have the lower frequancy.
The HS1100/HS1101 equivalent capacitor is charged through R2 and R4 to the
threshold voltage (approximately 0.67Vcc) and discharged through R2 only to the trigger
level (approximately 0.33Vcc) since R4 is shorten to ground by pin 7.
Since the charge and discharge of the sensor run through different resistors, R2 and
R4, the duty cycle is determined by :
thigh = C@%RH*(R2+R4)*ln2
tlow = C@%RH*R2*ln2
F = 1/(thigh+tlow) = 1/(C@%RH*(R4+2*R2)*ln2)
Output duty cycle = thigh*F = R2/(R4+2*R2)
To provide an output duty cycle close to 50%, R4 should be very low compared to R2
but never under a minimum value.
Resistor R3 is a short circuit protection. 555 must be a CMOS version.
So in other words, The circuit will charget the sensor 0.67*Vcc ( works as a capacitor )and it will change the output. Then it will start discharge via R2. When it will reach a level of 0.33*Vcc it will change the output agian. So infact you will get change of the output depanding on the capacitance of the sensor.
Forget about the formula, this are only given to you if you want to change the resistance.

Datasheet also talks about the STRAY capacitance.what is it?
also it is said to pay attention to soldering what soldering considerations should be there?

1. Well, When you build any PC board you will get STRAY capacitance and STRAY resistnace. They are asking to keep the lines between the sensore to other parts as short as possible, so the lines capacitance will not efect the sensore internal capacitance. for eaxample, if you wanted to have the sensor in one room and then pull the wires to the circuit on the other room, bad idea.
2. Soldering, bad soldering can have a large effect of the sensor. 2 examples:
A. It takes you for ever to solder a wire, you will effect or even damage the sensore because over heating the pin will also heat the internal sensor and can cause it damage!
B. Fast soldering or the solder iron not to hot, will cause a bad soldering point ( cold soldering ) and that can add resistance and capacitance ( sometimes ).
This are only few examples.
Read this 2 articles about soldering:

If you need more help, just post it here.
If you think i helped, clcik on the Helped me button.

Good luck.

Not open for further replies.

Part and Inventory Search

Welcome to EDABoard.com