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Opening 2 circuits with 1 signal

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fattytuna

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Hey Folks,
I've been lurking for a while, learning lots.

I have a project I'm working on and I can't figure out a good option.

The signal comes from a 555 setup in astable mode, 1 sec on 1 sec off. The output I want it to connect to control 2 circuits. 1 will ground a +12v and the other will power with +12v.

I've tried 2 pnp and they heat up and bunch of bad things happen. I also saw that a pnp and an npn combo could switch the circuits in alternating mode, which would be fine, but I couldn't set it up either. Both transistors are partially on when the 555 is in low. This is an issue I don't know how to fix.

So my next thought would be to put a dpdt chip to control the 2 pnp transistors.

Is there a simpler method?
 
How much current do the two loads require?
 
One circuit is a ground for a light, basically a switch for the light to turn on by allowing it to ground.

The other is a simple buzzer being turned on.

The reason I need these 2 circuits is that the first could be different based on the application. Some lights would be LEDs while others would be bulbs from 1.5v to 12v.

The buzzer pulls far less than 1 amp while the light could be as big as 1 amp.
 
The 555 output pin 3 is capable of sourcing and sinking ~200mA. When sourcing a few ma, pin 3 pulls down to ~0V and pulls up to ~11.5V. I conclude no driver is needed for the buzzer, only for the lamp.

Here is how I would do it: Note that the lamp resistance is so low that the voltage drop across it when M2 is off means that the buzzer gets almost full voltage.

I plot the voltage at pin3, the voltage at V(Xout), the current through the buzzer, and through the lamp.

47.gif
 
I'm going to ask you to search for "high side driver" and "Low side driver". They are not the cheapest solutions.

You might do well with two logic FETS with series resistors on the gates.

I can't quite figure if you desire the loads to be alternating or not?

The range of the 555 power supply matters most. Then the load power supplies which could be different. e.g. 12 V for the 555 but 5 and 24 VDC for the loads.
 
This is a circuit that plugs into a larger system so the inputs will vary. Because of that, I'd prefer to run FETS instead of directly sinking into the 555.

The 555 will be on a 12v power supply. The buzzer will be run off that same power source. The light has it's own voltage and it could vary depending on the system.

Mike, I forgot to mention that both circuits need to have an off mode when the 555 is turned off. The 555 is controlled by a switch too but that part I've already solved.
 
ou could just drive a relay and get up to 30 amps from the relay.
**broken link removed**
 
Here is a Toggle Circuit.
 

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Here's a quick diagram. I left the transistors as either pnp or npn, or there can be 1 of each. It sums up the basic idea while being faulty.
 

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Gee, I thought you mentioned FET's?

You MUST limit the base current. (Ib >= Ic(sat)/Hfe(sat.min)

Ib = Vin(base) =- Vbe

To put it differntly. Ic sat is the maximimum current that the collector can deliver. Hfe is the "current gain". At saturarion there is usually a different one. Grades of the transistrs have different ranges of Hfe. Thus, it has to work with the minimum value.

The minimun Vce or voltage between the emitter and collector also comes into play when driving a load. Using 0.6 to 0.7 is common.

FETs are a lot easier,but a small resistor is recommended for the gate like 100 ohms.

Now, the is something called a "digital transistor"
 
A lot of FETS are only available in a surface mount package, This **broken link removed** would work.

Why it works is 12 V is lower than Vgs max, Vds isn't exceeded, The FET will be saturated using a 12 V supply on the 555 timer. It has a low Vds on resistance.
 
Try this:

47a.gif

Buzzer R4 is on for 1sec, followed by the lamp R5. V2 can be any voltage from ~2V to about 15V
 
The IRF640N works, but overkill. Just remember to put like 100 ohms in series with the gate. It's a "real-world" thing you have to do, 99% of the time, it might work without it.
==
If you do drive a relay with your device, remember to put a reversed biased diode across the coil, close to the coil. This absorbs the voltage generated by the coil, when the coil is turned off which can damage the FET, in your case, driving it.
 
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