Delay 5V

masch · 04-09-2006, 12:17 PM

Hi

Trying to make a 5V delay circuit. I want the 5V rail to be delayed 10 secs after the initial 5V. Basically I'm trying to modify a usb port so that it turns on 10 secs after the PC is on.

I've tried with a combination of capacitors and npn transistors (9013) but when the output 5V turns on it becomes only 2.4-2.5V, I believe this is because the usb device itself has a resistance in it and the whole circuit becomes a divider.

I tried using a 5V relay to feed the 5V signal in, with the coil coming from a capacitor-based delay circuit to turn the relay on, basically what I did was when the capacitor charges to full (5V) the coil of the relay would turn on, but that's not what happens. The relay cant turn on this way.

I'm out of ideas, can anyone suggest how to turn on a relay after 10 secs?

spmclaugh · 04-09-2006, 12:39 PM

Quote: Originally Posted by masch

I've tried with a combination of capacitors and npn transistors (9013)

I think it would be helpful if you went into a little more detail as far as what type of circuit you made. Personally, I'd use a timer (mono-stable oscillator), maybe an inverter, depending on the devices purchased, and maybe an amplifier (to change line levels, if needed), and feed it into the npn transistor. You could use a relay too, using the same design, if you wanted.

The usual 555 monostable oscillator will go high after you feed it the trigger pulse, and this is the time you want the NPN transistor to be *off*. Depending on the RC values you chose for the mono-stable oscillator, it will go low after a certain time. Then you want the NPN transistor on, from now until forever (or at least until you turn your car off, or whatever). You'll have to look at what your NPN transistor characteristics to figure out what output voltage you want from this circuit (appropriate Vgs, or whatever your circuit needs).

masch · 04-10-2006, 01:16 PM

Not very good with electronics, and your comment about 555 timer chips, I wish you can write me a simple schematic so that I can understand what its about.

I'm only slightly familiar with transistors and capacitors. The pic below describes the intended circuit.

5V input from the USB charges up the capacitor. Ideally this should take 5 mins to charge it up. Once the capacitor reaches 0.6V, the left transistor turns on and connects ground to the right transistor's base, thus telling the right transistor to turn off. Upon turning off, the output of the circuit goes to 5V and goes to the USB device.

Unfortunately, it doesnt go to 5V because of the resistance in the USB device. Without the USB device I can measure it does go up to 5V in the output.

Alternatively tried doing the same with relays (with only one transistor, when base turns on, collector goes to 0. The Relay's coil is connected to +5V source and the other end of the relay coil connects to the transistor's collector). The idea is when the collector goes to 0, the coil will activate (because one side is +5, and the collector side is now 0) but it doesnt seem to work.

Can anyone suggest improvements? If you can teach me how to use the 555 timer chip that would be great also. Tutorial websites make it seem so complicated. I just need a simple circuit

mushin · 04-10-2006, 01:21 PM

How's this? http://circuitos.cl.tripod.com/schem/r84.gif

Of course, you'll want to throw in a transistor since I doubt the 555 can source your USB load.

BTW, here's a bunch more delay circuits. The above schematic is just the first link off the top. http://www.discovercircuits.com/D/delay.htm

masch · 04-11-2006, 02:48 AM

Thanks for the link. I also found one to drive a relay so that it can source my usb load.

http://www.aaroncake.net/circuits/relaytim.htm

Unfortunately I dont quite understand the 555 chip.

What is pin 2? It seems its connected to +V, and it will start when it goes 0V?

What voltage should pin8 be? 12V? 5V?

What's the voltage at pin3? Will it be equal to what's fed at pin8?

How do I make it start running starting from when the +5V line of the usb source goes live? Do I simply wire pin2 to ground?

What do you mean by throwing a transistor to source my usb load? Can I do that? I thought the transistor can only have an output of 0 or open circuit? The only way I know how to connect a transistor is with the Emitter at ground, Base detects voltage and Collector can be either open circuit or ground.

mushin · 04-11-2006, 02:19 PM

Can't answer your questions about the 555 off the top of my head - if you havn't already, try taking a look at the datasheet for one of them.

As for the transistor: an PNP or FET transistor could source the current, but that's not really what I meant. Just that you should use the 555 output to switch the USB current. The actual transistor could be downstream of the load as would be the case if you used an NPN transistor (which is probably the one you're familiar with.) The point is a transistor can handle alot more current than the 555. You could of course use a relay instead, but relays are clunky inefficient things.

shotgunefx · 04-11-2006, 02:39 PM

Quote: Originally Posted by masch

Not very good with electronics, and your comment about 555 timer chips, I wish you can write me a simple schematic so that I can understand what its about.

I'm only slightly familiar with transistors and capacitors. The pic below describes the intended circuit.

5V input from the USB charges up the capacitor. Ideally this should take 5 mins to charge it up. Once the capacitor reaches 0.6V, the left transistor turns on and connects ground to the right transistor's base, thus telling the right transistor to turn off. Upon turning off, the output of the circuit goes to 5V and goes to the USB device.

Unfortunately, it doesnt go to 5V because of the resistance in the USB device. Without the USB device I can measure it does go up to 5V in the output.

Alternatively tried doing the same with relays (with only one transistor, when base turns on, collector goes to 0. The Relay's coil is connected to +5V source and the other end of the relay coil connects to the transistor's collector). The idea is when the collector goes to 0, the coil will activate (because one side is +5, and the collector side is now 0) but it doesnt seem to work.

Can anyone suggest improvements? If you can teach me how to use the 555 timer chip that would be great also. Tutorial websites make it seem so complicated. I just need a simple circuit

Well in this specific schematic, 5v across 10Kohm will only give you 0.5milliamps for the load.

here's a good series of articles on the 555.