# Solving 555 Timer Issues with RH Over 45%

• GirishC
In summary, if you sweep the RH from 0% to somewhere 45% your 555 timer stops working. I have to replace the LCM555 timer and keep RH below 45%. I have a similar circuit with RA = 40.2K, RB = 360K and C is 105pF and centered at frequency 18KHz and it works very fine till 98%. I talked to National about this problem and they said that I can not use this chip beyond 5.6KHz (max). I verified the current requirements of discharge pin and trigger pin and what am I missing? The solution to my problem is adding another 555 timer.
GirishC
Hi

I am currently using LCM555 timer for generating variable frequency based on relative Humidity (RH) of surrounding. The LCM555 is configured as astable multivibrator and centered at 20KHz. RA = 24K, RB = 200K and C is capacitive sensor having value of 160pF at 30C, 0%RH. The C is given by formula

C = C0*[1 + HC0*RH]

C0 = 160pF at 30C, 30%
HC0 = 3420ppm/%C
RH = % relative humidity

temperature derating factor is
dC= - 0.0019*(T-30) pF
T = temperature in C

When I sweep the RH from 0% to somewhere 45% my 555 timer stops working. I have to replace the LCM555 timer and keep RH below 45%. I have similar circuit with RA = 40.2K, RB = 360K and C is 105pF and centered at frequency 18KHz and it works very fine till 98%.

When I talk to National about this problem, I received a feedback saying I can not use this chip beyond 5.6KHz (max), according to datasheet. I have queries based on my experiment

1. How can upper limit being 5.6KHz is possible. We have many application where 555 used goes much above 100KHz.
2. What causes failure of LCM555 and why it works in other configuration? I have verified the current requirement of discharge pin and trigger pin. What am I missing?
3. What is the solution to my problem.

Could anyone through light on this?

Girish

What happens if you change the ratio between Ra & Rb (increasing Ra and decreasing Rb)?

With your circuit you will be measuring humidity by measuring the frequency, correct? Wouldn't it be easier to use one 555 running at a constant frequency to trigger a second one as a monostable The averaged output voltage (pulse width) would then indicate humidity.

Thanks for input

The operating frequency is between 10KHz to 100kHz, recommended is 20KHz. So I chose the values closest to 52% duty cycle. I checked with RA = 40.2K and RB = 360K and it failed at 70% RH. To keep accurate RH calculation I chose these resistors in the range of 0.05% so I have upper limit on the resistors.

My worries are, 555 chip is physically damaged at these values so reducing the Resistor will worsen the problem ( I feel so)

Adding another 555 timer will increase board size and will add up to the cost. Also I will have to redesign the complete system but I am not sure is it worth to take these efforts.

## 1. How does the RH over 45% affect the 555 timer?

The RH (relative humidity) over 45% can cause the 555 timer to behave erratically or malfunction. This is because high humidity can cause moisture to accumulate on the electronic components, leading to corrosion and short circuits.

## 2. What are the common issues encountered when using a 555 timer with high RH?

Some common issues that may occur when using a 555 timer with high RH include incorrect timing, unstable output, and failure to trigger or reset properly. These issues can result in the overall failure of the circuit.

## 3. How can I protect my 555 timer from high RH?

To protect your 555 timer from high RH, you can use a conformal coating or a moisture barrier to prevent moisture from reaching the electronic components. It is also important to keep the circuit and components in a dry environment and to use high-quality materials and components.

## 4. Can I use a 555 timer in high RH conditions?

While it is not recommended to use a 555 timer in high RH conditions, it is still possible to do so with proper precautions. As mentioned before, using a conformal coating or moisture barrier can help protect the timer. Additionally, you can also use a dehumidifier in the surrounding area to reduce overall humidity.

## 5. Are there any alternatives to using a 555 timer in high RH environments?

Yes, there are alternative timer circuits that are specifically designed to operate in high humidity environments. These circuits use different materials and components that are more resistant to moisture and can provide more reliable performance in these conditions.

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