A (reverse?) Time Delay

[kage]

I am attempting to put a time delay on a switch, but in sort of a funky way. I am trying to mimic a power on switch, but one that must be held ON for a certain amount of time (ie. 3 seconds). If it is held on for less than this, nothing happens and no voltage is supplied to the load. If you hold the switch for longer than required (and any length after that) voltage will be supplied to the load. My first attempt was a monostable multivibrator, but it was fruitless. I've also tried some simple RC circuits, but they don't work quite right.

Any help would be appreciated.

ps. The application for this is a 'child-safe' or 'bump-resistant' switch.

 

[blueroomelectronics]

Almost any microcontroller would work.

 

[shaneshane1]

I made something like this not that long ago, i used a simple RC circuit with a LM339 comparitor i used the switch in between the resistor and the cap, so when i pressed the button down it took eg: 3 seconds to charge the cap to 2/3, and once it hit that voltage in the IC (which you need to set) the output would activate.

Hope this helps you!!!

 

[Diver300]

What do you want to make it to turn back off again?

Your circuit needs to remember that the button has been pressed for long enough some time in the past. Therefore it will stay on. To be useful, something will have to get it to turn off.

 

[Boncuk]

Reversed time delay

Hi kage,

you can make the circuit using an OpAmp wired as a comparator. The threshold point is set at the inverting input. Pressing the ON-button the electrolytic capacitor will charge until the voltage at the noninverting input exceeds the threshold voltage. Resistor R6 works as a positive feedback to compensate for losses (self discharge) of the cap. After some time the voltage at pin 3 will be almost equal to the output voltage of 10.5V

With the parts dimensioned (standard R-values) as in the schematic the delay is about 3 1/2 seconds. Adjusting the voltage divider at the inverting input you can achieve an accurate 3 seconds delay. Use a trimpot and a lower value resistor in series for fine adjustment instead of 150K for R2 (100K fixed value and 50K trimpot).

Since you certainly want to switch off without delay use button OFF to discharge the cap rapidly. The circuit switches off with no delay. For delayed turnoff use a higher value discharge resistor (R5).

Regards

Boncuk

 

[shaneshane1]

Like i said the LM339 comparitor will do that job for him, he can set the RC time for 3seconds for the switch to come on, and for the off time, just hold it down again until it goes off (3seconds) and so on.

 

[Boncuk]

Like i said the LM339 comparitor will do that job for him, he can set the RC time for 3seconds for the switch to come on, and for the off time, just hold it down again until it goes off (3seconds) and so on.

Hi shaneshane1,

I think he was looking for a circuit. May be he is new to this. So I just gave a little advise.

Regards

Boncuk

 

[Hero999]

A 555 with an inverter and an NOR gate on one input on the output of the 555 and the other on the switch will work.

When you hold the switch down the trigger will go low and the 555 will output a high for a certain period of time. The invertaer's output will be low for this period of time and so will the switch so nothing will happen. The NOR gate will only be high whilst both the 555 and the switch are low.

A one chip solution might also be possible using a quad NOR with two configured as a monostable flip-flop.

 

[saturn1bguy]

Here's Boncuk's circuit with a triac output in case you're wanting to drive something line-powered with it.