[SOLVED] First 555 project question

[poolshark021]

I'm trying to design a circuit to automatically turn on a tv when it is plugged in. My tv power button won't work for the first 4 seconds after it's plugged in. I need a circuit that waits 5 seconds after being powered on and then provides a 1-2 second pulse to ground to simulate the power button press and then never again (a second pulse would turn it off). I'm trying to use a 555 timer in monostable mode but I'm unsure about the delay to start the trigger. Here is what I've got so far. (The program I'm using doesn't show GND pin on 555 for some reason)


My question is that the 555 datasheet I'm looking at says the trigger pin has to go back HI before the threshold voltage is reached in order for the output to be turned off. The way I'm reading this the output would never turn off since the trigger pin would always stay low once C1 was charged? Do I need to drain C1 during the output?

 

[betwixt]

This isn't normally how you would do it but it depends on exactly how the TV works.
If all it needs is a delay, you can use a simple monostable circuit (yours is an astable or oscillating circuit). This would effectively hold the TV switch low for say 5 seconds then release it.
If the TV needs to initialize with the switch NOT pressed then you want to give it a single press after say 5 seconds, you need a second monostable, another 555 or a 556 which has two in one package.

Please try this so we know how it needs to operate:
Hold the button down while you plug in and keep it down. See if the TV starts up normally after a few seconds.
If it does, you need a small change to to your schematic. If you have to release the button and press it again before the TV starts, you need that second monostable.

Brian.

 

[poolshark021]

Yes I’ve tried this already and the switch can’t be held down when the power is turned on. It needs to wait 5 seconds and then have a single press

 

[betwixt]

OK, you need two monostables, the first is to provide the delay after turning the power on, the second is triggered by the first and provides the 'push' signal from the button. The first timing is already known, it has to be about 5 seconds, the second may need some experimentation but probably should be about 1 second. I have a Panasonic TV that requires the button to be held for 3 seconds (yeah- strange but true) so you might have to extend the timing a little.

I suggest you download the data sheet for the 555 and 556 devices as these have the application notes that show how to combine the monostables and also the formulas for calculating component values.

Brian.

 

[poolshark021]

I have the data sheet but I'm still unsure about starting the first trigger. The cascading diagram still shows a momentary switch to start it off. I need it to trigger as soon as the power is on and then go high to release the trigger

 

[betwixt]

It is actually very simple, all you need is to delay the trigger going high for a while after the power is applied. The trick is to use a resistor and capacitor network, you have the right idea in your first post but the resistor and capacitor are swapped. The values depend upon the rise time of the supply, the capacitor charges slowly through the resistor so it follows the supply voltage with a delay. A higher value resistor or capacitor will delay for longer but the period is fixed by the values, if the supply rises very slowly you need to increase the delay or they will rise too closely to each other. I would suggest a resistor from trigger to supply of about 10K and a capacitor of about 0.1uF as good starting values.

Brian.

 

[poolshark021]

Ok that makes more sense. Does this circuit look right to you? Thanks for your input!

 

[d123]

Hi,

Just to add a different idea, a discrete version could be made with a quad transistor array or 4 individual NPNs... Probably +- same number of components as two 555s.



Just a thought for another occasion.

 

[poolshark021]

Hi,

Just to add a different idea, a discrete version could be made with a quad transistor array or 4 individual NPNs... Probably +- same number of components as two 555s.


Just a thought for another occasion.

Thanks for the idea! I was thinking there could be a way to do it with just transistors but I'm not at that level yet haha

 

[betwixt]

Ok that makes more sense. Does this circuit look right to you? Thanks for your input!

That looks good to me! The first monostable should hold off the second one by about 5.2 seconds and the output pulse should be about 1 second.
The resistor marked "33" should be "33K".
As a precaution against noise or supply instability, it would be a good idea to add a capacitor of about 100nF directly across the supply pins of the 555s, they are not essential but it would be considered good practice.

d123's circuit should also work, it uses a different principle, two timing circuits running simultaneously, one for about 6 seconds and one for about 5 seconds with the output being the difference between them.

Brian.

 

[poolshark021]

That looks good to me! The first monostable should hold off the second one by about 5.2 seconds and the output pulse should be about 1 second.
The resistor marked "33" should be "33K".
As a precaution against noise or supply instability, it would be a good idea to add a capacitor of about 100nF directly across the supply pins of the 555s, they are not essential but it would be considered good practice.

d123's circuit should also work, it uses a different principle, two timing circuits running simultaneously, one for about 6 seconds and one for about 5 seconds with the output being the difference between them.

Brian.

Great I appreciate the help! I realized the 33k typo after i posted but it was too late to edit or cancel. My parts showed up today so time to break out the breadboard and test it out. Thanks!