Whats wrong with my fish tank auto illumination circuit

Thread Starter

Tarek Hossain Dinaji

Joined Apr 2, 2015
8
When I power it up it works just fine. It responds to the light. But after it is working about 20 minutes when I shadowed the ldr it switched off the relay but when I removed the shadow it does not activate the relay anymore. Now when I switch off the whole thing for 30 minutes it again starts and then after 20 minutes if I cover the ldr same old story.

What is wrong with this circuit. How can I get a reliable service?

Thanks in advance.
 

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cmartinez

Joined Jan 17, 2007
6,467
When I power it up it works just fine. It responds to the light. But after it is working about 20 minutes when I shadowed the ldr it switched off the relay but when I removed the shadow it does not activate the relay anymore. Now when I switch off the whole thing for 30 minutes it again starts and then after 20 minutes if I cover the ldr same old story.

What is wrong with this circuit. How can I get a reliable service?

Thanks in advance.
Are any of the components warming up?
 

Thread Starter

Tarek Hossain Dinaji

Joined Apr 2, 2015
8
My concern is the 300k Resistor. I did not have a 300k available so I solder a 330k.

Most of the time I saw pin 5 is partially grounded via 0.1 / 0.01 uF capacitor but in this diagram it is sorted to pin 3
 

pwdixon

Joined Oct 11, 2012
488
The 330k resistor is providing some hysteresis to the comparator so it isn't going to make much difference what value it is.
Not really making much use of the features of a 555 as a timer you are just using the comparator, you might have understood he circuit better if you simply used a comparator instead.

Have you checked your battery voltage when the circuit doesn't switch?
 

SgtWookie

Joined Jul 17, 2007
22,201
I don't see any capacitors across the 555's supply pins (4 and 1).
The minimum requirement is one 0.1uF ceramic or poly metal film capacitor, and one 1uF or larger aluminum electrolytic capacitor in parallel.

What is the current requirement of your relay's coil, or what is it's resistance?
The BC547B is only rated for 100mA maximum collector current; best to not push it past 70mA max. The 555 (transistorized) can actually sink or source up to 200mA directly from it's output. It's better to sink current with it, as maximum source output voltage is ~1.3v less than Vcc.
What are you using for a power supply?
 

SgtWookie

Joined Jul 17, 2007
22,201
One rather large problem with that circuit is many common comparators like the LM311, LM339, LM393 have open-collector outputs which can sink, but not source current - which would make that circuit completely non-functional. Certain opamps might be used, but some (like the ancient 741) might not ever let the transistor Q1 turn off due to the output not getting within about 2v of either power rail. R2 being 10k would severely limit Q1's base current, so Q1 wouldn't saturate; the collector-emitter voltage would be so high that the relay wouldn't turn on. If the transistor did manage to turn on, and then off, Q1 would be blasted to smithereens because there is no diode across the relay coil to provide a current path for the reverse EMF which will occur when Q1 turns off. Finally, there is no feedback to provide hysteresis, so when lighting is near the trip point, the relay would chatter incessantly.
 

Thread Starter

Tarek Hossain Dinaji

Joined Apr 2, 2015
8
I don't see any capacitors across the 555's supply pins (4 and 1).
The minimum requirement is one 0.1uF ceramic or poly metal film capacitor, and one 1uF or larger aluminum electrolytic capacitor in parallel.

What is the current requirement of your relay's coil, or what is it's resistance?
The BC547B is only rated for 100mA maximum collector current; best to not push it past 70mA max. The 555 (transistorized) can actually sink or source up to 200mA directly from it's output. It's better to sink current with it, as maximum source output voltage is ~1.3v less than Vcc.
What are you using for a power supply?
Thanks for the reply. I appreciate it very much.

The Relay is rated for 12V and 12A and the shopkeeper said this is IC 555 and IC 1408 Version. I am outside now so I cant check the resistance.
 

Thread Starter

Tarek Hossain Dinaji

Joined Apr 2, 2015
8
remove R2, its not required..the 555 has built in hysteresis of 1/3 and 2/3 supply, the light comes on when the ldr sees daylight and goes off when in darkness.
Thanks.

I will try removing it and ground it using 0.1uF. See how it deals with that. I intentionally added the transistor to keep the IC cool as It will work continuously for months and years.
 

SgtWookie

Joined Jul 17, 2007
22,201
If you're going to use a transistor, then use a 2N2222 or PN2222, as they are rated for a maximum collector current of 800mA (500mA is a practical design limit) rather than 100mA (~65mA practical) like the transistor you are using. The 555 timer itself has a rating of ±200mA from it's pin 3, about 3x that of your transistor. It really does help to read and understand datasheets. When you're starting out, datasheets can seem intimidating, and a practically incomprehensible mountain of information - but feel free to ask questions. Nobody starts out knowing this stuff.
 
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