Jittering state relay

[ABR2]

I am looking for some help regarding how to control a 12V 5pin relay switch to stop jittering when it reaches its critical position of ON to OFF.
The relay is controlled by a LDR and BC 547.
Is there any other components that need to be added to get a clean change of state.
The project is to control a outdoor pump thru LDR

 

[FvM]

Need to implement circuit with positive feedback and different on- and off-thresholds (Schmitt-trigger). A second transistor is required.

 

[KlausST]

The relay is controlled by a LDR and BC 547.

--> show your circuit. Including all part values.

What light source is the LDR exposed to? (complete darkness, sunlight, artificial light like LED, neon ...?)

How do you expect the circuit to operate?

Klaus

 

[ABR2]

The circuit is for an 12v outdoor pump working during the day and switching OFF for the night.
The daylight will control the LDR which in turn will be controlled by a variable potentiometer.

FvM: do I need a positive feedback and/or a second transistor would do the job.

 

[FvM]

You need positive feedback, at least a second transistor is required to implement it.

 

[betwixt]

... but probably better to use a dedicated comparator IC.

The fundamental problem is that you need an abrupt turn on and turn off but the simple LDR/single transistor is giving you a more gradual change in relay coil current.
As the current increases enough to make the relay 'just' operate, it is particularly prone to noise and power supply variations and those are likely to be coming from the pump itself.

The feedback FvM mentions is a method of setting two thresholds, one to turn the pump on and another to turn the pump off. A higher voltage is needed to turn the pump on than is needed to turn it off so the 'jitter' current is lost between the upper and lower voltages. Typically, the output state is used to change the threshold so it automatically switches from one to the other.

Brian.

 

[danadakk]

Make sure your supply rail has very low noise on it. That can cause relay chatter.

Picture of your layout, and a schematic might be useful for forum to give you
help. Part numbers as well.

 

[ABR2]

Thank you all for your input. Attached is my schematic layout of my project and a picture of my relay
My supply rail is from a 12 VDC little transformer set on 9V.

 

[KlausST]

Hi,

electronics design means using values and math .. a picture can´t give the necessary informations.
--> better post a link to the relay datasheet. .. and a link to the LDR datasheet.

You can easily reduce the file size of the schematic to less tahn 50kBytes without losing thread related informations.

****
This seems to be your current, jittery schematic.
I miss
* a power supply capacitor
* a noise reducing capacitor aross B-E of the BJT
* a freewheeling diode across the relay coil
* in case the load acts like an inductance: snubber or freewhelling diode across the relay switch
* the information how you generate 7V from the 12V
* a picture of the Layout as requested
* information which LDR exactly is used

***
You did not yet include any of the given recommendations.

In case you don´t know what to do: just do an internet search for "two tranaistor LDR relay circuit" .. and many suitable solutions will show up.
Enough to at least give an idea ...

Klaus

 

[betwixt]

100% agree with Klaus and the fact you power the load from the same supply is increasing the risk of jitter. As the load turns on (relay contacts close) the supply voltage almost certainly dips under the extra load and that turns the relay off again, hence the jittering.

The transistor and LDR are both at risk of being permanently damaged too. Imagine you turn the potentiometer to minimum resistance so it is basically a short circuit from end to end and the LDR is illuminated so its resistance drops, as it would be directly across the 12V it could pass sufficient current to burn itself out or at least get hot. Second problem is as Klaus pointed out, you need to place a diode across the relay coil, cathode end to the supply. It will not normally conduct but when the relay is turned off, the stored magnetic energy in the coil will be released and in the opposite polarity to the way it was applied. This could cause a significant voltage spike on the transistors collector and zap it permanently.

Simple fixes:
1. Add a fixed resistor in series with the LDR to limit the current that can flow through it to a safe amount. We need LDR characteristics to advise the best value.
2. Add a diode, 1N4001 for example, the type isn't critical, across the relay coil.

Then ideally look at using a comparator circuit and adding hysteresis to the trip point by adding some positive feedback.

Brian.

 

[FvM]

Example of relays driver with hysteresis.

 

[danadakk]

Another possibility :

555 IC’s Hysteresis for Dark Activated Relay – Electronic Circuit Diagram

Ignore the use of opto isolator. Output relay with protection diode across it. Add a bypass
cap on 555 timer power pins, a .1 uF ceramic and a 10 - 50 uF electrolytic.

What is the LDR part number ?


Regards, Dana.

 

[ABR2]

Thank you all for your guidance and advice.
I will try later the other two proposed options

Here is the data from my large LDR:
Cadmium sulphide (CdS) light dependent resistor cell.
- Dark resistance: 0.5M ohm min.
- Light resistance: 2.8K ohm min 8.4K ohm max.
- Max dissipation: 100mW
- Rise time: (0-63%) 50mS
- Decay time: (100 - 37%) 40mS
- Dimensions: 8.5mm x 10.14mm
- Lead spacing: 7.68mm.

 

[ABR2]

After many attempts, I finally succeeded at removing the jittering on the relay by using, as suggested, the "CA3140 Wideband Op-Amp Linear IC"
Just wanted to share with you my success and thanking you all for your advice.
Regards.