Integrated Circuit Selection

[pbhuter]

I took a class in digital circuits a long time ago, and I'm sure this can be done, I just don't know how:

I want to run current from a battery (say a standard 9V battery), then switch to pull current from another source. I want to continuously alternate between these two "states", and I imagine an IC would be the way to do it, I just don't know which IC to use.

So - Can anyone tell me which IC to use?

Thanks!

 

[sky_123]

Search for 555 on Wikipedia, or here.
it has enough power to drive (say) a small LED, at speeds from (say) once every few seconds to a few hundred kHz
(and sometimes outside these ranges too).
To drive a heavier load than an LED, the output from the 555 can be used to control a separate BJT or MOSFET or a relay.
The link mentioned has lots of useful 555 circuits, and also a table to calculate what resistor/capacitor values are
needed for a given freq (the wikipedia page has the formula too). Search the link shown above for 'relay' and there are
some examples showing how to interface that. Search that page for "PWM controller" and it shows how to use a MOSFET too.

 

[pbhuter]

Awesome! Thank you.

- - - Updated - - -

UPDATE: I'm looking through the Wikipedia article, and glancing at the PDF of the link you posted. Based on what I see on Wikipedia, I can easily build something to pulse an output (Wikipedia has words to go with the pictures, so it's a bit easier on a "first look" basis). Do you know if the PDF link or Wikipedia has something that will let me pulse, then allow (for simplicity) a capacitor to discharge (probably on a different pin)?

Thanks again for the reply, and any future assistance.

 

[BradtheRad]

Here's a schematic showing how transistors might be used.

The clock can be generated by a 555 as sky_123 describes.

For simplicity this layout uses a common ground, and a mix of NPN and PNP type.

The NPN has the bias going down through the load. If your load needs greater current then you may need to use all PNP's. In that case one of the clock signals will need to be inverted.

 

[pbhuter]

Here's a schematic showing how transistors might be used.

The clock can be generated by a 555 as sky_123 describes.

For simplicity this layout uses a common ground, and a mix of NPN and PNP type.

The NPN has the bias going down through the load. If your load needs greater current then you may need to use all PNP's. In that case one of the clock signals will need to be inverted.

Is the "load" my discharge (represented by a 50k resistor)? Where do I connect to pulse my "charge"? I think I can get the 555 set up by looking over all the stuff from sky_123 - do I just connect the output (say, where I would connect an LED) to the CLK on your drawing? What are the different voltages at the top?

I am sorry that it probably sounds like I have no idea what I'm doing, and I appreciate any help.

 

[sky_123]

I'm not 100% sure about what you want, but it sounds like you're asking for a so-called 'half-bridge'. It can be built
from discrete components with some care, or something like L293 ic can be used (it contains 4 of them - you only need 1).
It basically looks like two semiconductor devices, one on top of the other. Depending on which device is turned on,
the common connection between the two devices will either be close to the supply rail or close to zero volts.
With the half-bridge, the two devices have their inputs connected together, such that a high turns on one device, and a low
turns on the other. There is a bit of care involved in the circuit to ensure that only one device is turned on at a time
during the transition between low and high (otherwise there would be almost a short across the supply). The L293
incorporates that.

You also need to ensure there is sufficient series resistance to the device you're charging/discharging,
otherwise it could damage the semiconductor devices.

 

[BradtheRad]

Is the "load" my discharge (represented by a 50k resistor)? Where do I connect to pulse my "charge"? I think I can get the 555 set up by looking over all the stuff from sky_123 - do I just connect the output (say, where I would connect an LED) to the CLK on your drawing? What are the different voltages at the top?

I am sorry that it probably sounds like I have no idea what I'm doing, and I appreciate any help.

Sorry, I incorrectly thought you want to power a device first from one supply, then a second supply.
I didn't realize you want current to go one way, then the other.

Going along with sky_123's post #6, this is a simple layout for charging and discharging a capacitor.



CLK (clock) is a repetitive signal that controls the action. A 555 IC would be a good choice for this (as sky_123 suggested).

First the PNP transistor turns on, while the NPN is off. Then the opposite occurs.

A resistor is in series with the capacitor, to limit current.

The power supply is labelled at the top. 5V can be changed to whatever you wish.

 

[pbhuter]

Sorry, I incorrectly thought you want to power a device first from one supply, then a second supply.
I didn't realize you want current to go one way, then the other.

Going along with sky_123's post #6, this is a simple layout for charging and discharging a capacitor.



CLK (clock) is a repetitive signal that controls the action. A 555 IC would be a good choice for this (as sky_123 suggested).

First the PNP transistor turns on, while the NPN is off. Then the opposite occurs.

A resistor is in series with the capacitor, to limit current.

The power supply is labelled at the top. 5V can be changed to whatever you wish.

Okay, I just want to make sure I have this down:

The 555 will require a different power supply (+ connected to the +5v in the drawing, and - connected to the ground in the drawing) because the CLK is just the output from the 555, and the 555 will be configured based on what sky_123 sent me?

I think that's the last question, again, thanks for the help.

 

[sky_123]

Hi, you can power the 555 and Brad's circuit from the same supply. The output of the 555 (pin 3 on the chip) goes to the 'CLK' pin marked on the circuit.

 

[BradtheRad]

Okay, I just want to make sure I have this down:

The 555 will require a different power supply (+ connected to the +5v in the drawing, and - connected to the ground in the drawing) because the CLK is just the output from the 555, and the 555 will be configured based on what sky_123 sent me?

I think that's the last question, again, thanks for the help.

It is necessary for the 555 to be powered from the same supply. Its output needs to be referenced to the same supply as the transistors, so it can turn them on and off.

It so happens this simulator program supports the 555 chip. Here's an updated diagram: