Reading light delay circuit

[msrtlt]

Hello,
I tried (unsuccessfully) to find a schematic for a reading light delay circuit that would operate as follows: it will opearate from 120 volts AC and instead of an on/off switch would have a push button. When the button is briefly pressed, the light will go on immediately and will stay on for say, 30 minutes. After this delay the light should go off. If you want to continue reading, just press the push button again for another 30 minutes and so on. This system is to prevent the light being on for a long time (or the whole night) after I've fallen asleep during bed time reading...
Any ideas would be greatly appreciated! I have some knowledge of electronics but could not come up with anything simple - ideally there would be no ICs and a triac will be used to switch the lamp.

Even better, it would be nice to have the lamp slowly dimm off as opposed to going off suddenly at the end of 30 minute interval... But I would settle for the first scenario - initially...

 

[Mikebits]

Something like so could work.
https://www.elecfree.com/circuit/IC-555/555-timer-5-to-30-minute/
For the relay use one that has contacts rated at AC voltage. The relay could be wired so if normal lamp switch is on the timer is ignored, or you can flip the switch and engage your timer, or something like that. You could also do away with the range option and set the timing to one point. I have not thought about it much.
Just google 555 timers for more info.

 

[things]

There are actually some switches that have a delay like that. I'm not sure what they are called, but you push them, and they slowly come back out, and cuts the power when they are fully out. If you can find one it will be much easier ...

 

[Mikebits]

I have seen time delay relays, but they are not cheap, at least from what I have seen.

 

[msrtlt]

I rather not use ICs, as they require some sort of DC power supply and the circuitry always draws current. What I am after is a triac circuit with perhaps a couple of transistors and a capacitor charging/discharging circuit - ie push the button - the capacitor start charging/discharging (depending on the circuit) and eventually cuts the triac off...

The kind of switches that slowly come back after being pressed are usually pneumatic, bulky, wall mounted and expensive.... so here!

 

[Mikebits]

**broken link removed**

 

[msrtlt]

This looks like a possible candidate - thanks.
It would be more exciting, though, to build my own - if you stumble upon an adequate schematic, I'd appreciate if you let me know!
Thanks a lot for the link.

 

[Hero999]

The 555 circuit would work.

Just wire the 555 so it turns on when the power is applied and the relay turns it off the power to the transformer when the delay expires.

T1 is a mains transformer with about output voltage between 6V and 9V.

C is a tantalum capacitor.

If C = 47:mu:F and R = 33M the delay will be about 28 miniutes but in practise it'll be longer due to the leakage current.

You could easilly replace the transformer, rectifier and filter capacitor with a wall plug.

 

[k7elp60]

If you did not want to use a relay as in hero999 circuit, you could use a triac and a triac driver such as a MOC3021.

 

[msrtlt]

Thanks a lot for the schematic. It will definitely do the job. What I like in particular, is that the circuit does not draw any current when in off position. I will try to build it.

As far as substituting the relay with a triac, I am not sure how would that work, as the relay contacts are in parallel to the push button - this configuration allows the 555 to be supplied after the push button is released. If you have a particular configuration in mind, I'd like to see it.

Again, thanks a lot!!

 

[k7elp60]

Thanks a lot for the schematic. It will definitely do the job. What I like in particular, is that the circuit does not draw any current when in off position. I will try to build it.

As far as substituting the relay with a triac, I am not sure how would that work, as the relay contacts are in parallel to the push button - this configuration allows the 555 to be supplied after the push button is released. If you have a particular configuration in mind, I'd like to see it.

Again, thanks a lot!!

Here is a suggestion.

 

[msrtlt]

Thanks Hero999 and k7elp60!

I can combine both circuits now. One question: I am not sure I understand why does one need the opto-coupler - can't the triac be driven directly?

Also, since we now have a triac, is it possible to make the light to go out gradually as opposed to switching off immediately?

 

[Hero999]

Generally you should always put the switch in series with the live (hot) conductor for safety reasons, however as this switch isn't supposed to fulfil a safety critical role it doesn't matter.