Activate circuit with small delay

[nostra16]

Hello,

I'm a bit new to electronics, trying to get some knowledge next to my electromechanics, and want to build what I thought was a simple circuit.

I want to activate a circuit (like a row of leds) a little time (like 4 seconds) after I establish current in the said circuit.

My first idea was to use a RC cell directly on the transistor, but I had a slow power up of the leds, and I don't want that.

Next step was to use a schmitt trigger to have a clean transition between the off and on stage. I bought a few SN74 triggers and started testing today, but what I measure does not fit with what I understood of the schmitt triggers.

My observations are : when I let the input float, it sets itself at 1,6V and leavs the output at 0V. When I apply tension on the input, the output stays at 0V, normal as the trigger is also inverter. The only way to get tension on the output is to force the input at 0V by directly connecting it to the ground.

I am therefore lost. How can I activate the trigger using an RC cell? I put a diode between the cell and the input, avoiding the input to charge the RC cell. But I can't seem to force input to 0V using the cell. Will adding a PNP transistor between the cell, the input of the trigger and the ground help?

I am very new to Eagle (15 minutes ;o) but I took a shot at two diagrams to better understand. Set 1 is the one I tested, but is not working. Set 2 is an idea I had this evening, not got a chance to test it yet.

Thanks a lot for reading me, and thanks in advance for any help/ideas!

nostra16

 

[jimmythefool]

Have you looked at a 555 time delay circuit? There are 100's of examples on the net. Simple to make, and adjustable too.

 

[crutschow]

TTL logic requires that you sink current from the input to force a low. If left floating it goes to a logic high as you observed. If you look at the data sheet you will see the current required to sink for a logic low (it can be as high a 1.6ma depending upon the TTL version you have). Thus the diode you added doesn't work because this current can't flow.

The second circuit should work if you remove the diode so the transistor base current can flow (PNP base current flows out of the base). With 10kΩ for the resistor, somewhere around 500µF should give you a 4s delay.

But as jtf suggested, a 555 circuit is probably easier, if you can do that.

 

[nostra16]

Thanks for the very fast reply!

I will test the second circuit, for leaning purpose. I have ordered a bunch of 555's that should arrive next week, as I am planning to build an astable circuit as well. Seems it would have been better to have 556's to combine monostable and astable in the same chip.

Found lots of references with "555 time delay circuit", thanks for the tip.

nostra16

 

[nostra16]

I tested this layout this morning, and to my big surprise it worked!

I don't really get the purpose of R3, but without any resistor the transistor fried. I had to throw out a few before realizing that was the problem ;o)

If anyone can briefly explain why R3 is important, plus any simplifications I could to...

A weird thing I noticed : the capacitor does not always discharge when I remove Vs (leaving the ground). Any way I can be sure the cap will discharge fast once the source is cut?

Thanks,

nostra16