power supply question

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firealarmfrea785

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Hey, i have a 555 timer wired up in bistable mode for a latch. It works fine the problem is:

I am running a fire alarm strobe on 18 volts, but the 555 timers max is 16. The strobe needs at least 18 to work correctly.

I am having trouble using my 18 volt power supply for my strobe and the 3 volts for the 555 timer.

Basicly when I connect the 18 volts, it triggers the circuit and when I hit reset the strobe does not turn off, even though the timer resets.

I need to know how to connect it to an external power supply, so the strobe can run on 18 and the 555 can run on 3-9 volts dc.

Chris.
 
Use an LM7812, it will be the easiest solution. Put it between the power supply and the timer, and power the strobe before it.
12VDC Linear Regulator, Maximum 25V Input. There is a 5V version available as well LM7805, but you will generate excess heat if you use this model. The 555 shouldn't complain with 12.
 
Hi Firealarmfreak,

How are you switching the stobes? Are you trying to drive them directly from the 555 or are you using transistors or relays or something?

Can you post the schematic?


Torben
 
Now that you mention it Torben... Good point...
It would fail miserably if OP was trying to drive it from the Q of the 555...
 
Hey, Here I used this schematic: PS, I did not make the schematic, it was on a website. This schematic is a 555 timer in bistable mode.

I connected the output of the 555 timer to one side of my strobe, the other side of my strobe into the negative 18 volt power supply, and the positive 18 volt into pin 1 on the timer and i even tried pin 6, both gave the same result.

PS. If i connect an led from the 555 timers output and the other end into ground on the breadboard (555 circuit) it works fine, but i cant do that for the strobe, cuz the leds, timer and strobe operates on different voltages.

**broken link removed**
 
firealarmfrea785 said:
Hey, Here I used this schematic: PS, I did not make the schematic, it was on a website. This schematic is a 555 timer in bistable mode.
Click to expand...

Thanks, but that's just a schematic for a 555 circuit; I meant the schematic of what you actually have wired up, including supplies and the strobes etc. No matter, you explained it below anyway.

I connected the output of the 555 timer to one side of my strobe, the other side of my strobe into the negative 18 volt power supply, and the positive 18 volt into pin 1 on the timer and i even tried pin 6, both gave the same result.
Click to expand...

The 555 can't drive that kind of load directly. Use output transistors and/or relays to drive the strobes; that way the lamps don't have to care what the voltage on the 555 is and vice versa.

Also do what erosennin said--18V is the Absolute Maximum supply voltage for the 555 which means you don't want to run it at that for very long. 12V should be fine.


Torben
 
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Could i just use a transister as the 555 timers output and connect the strobe to that? If not how do i connect the regulator? can you try to explain?

Thanks for all your help so far.

Chris.
 
Can someone give step by step instructions on connecting a voltage regulator so the 555 timer can be run at 5 volts and the strobe can be run at 18 volts, but the 555 timer can still control the strobe (my case latch, cuz the timer is in bistable mode).

A schematic would do also, Im using a breadboard with + and - power strips on both sides of the breadboard. I have my 555 timer circuit hooked to the + and - strips on one side.

As far as i understand from the replies, a rehulator is what i need, but im confused on how to connect it. Thanks.

Chris.
 

Why are you fixated on 5V? 12V is easier to attain given that you have an 18V supply already. Hook up a 7812 with the input to the 18V line, ground pin to ground, and the output pin to the 555 supply pin. From the LM7812 datasheet, you should put a 0.33uF cap from the 7812 input pin to ground and a 0.1 uF cap from the 7812 output to ground as well.

I think that should do the trick.


Torben
 
i wander why don't you put diodes in series with the lm555 3 will do it 1n4001 why regulate anything all we need is limit the voltage to 16v simplicity is the mother of all vertue.
 
That would work but the absolute maximum voltage of the 555 is 16V and three diodes would drop the voltage to 16.2V. Absolute maximum ratings are not normal operating conditions - you shouldn't push a device to it's absolute maximum ratings or you're asking for trouble. The maximum normal operating voltage of a 555 is 15V, operating it continiously from higher voltages isn't recommended.

Don't get me wrong suppose you want to use your 555 in an application powered from 10 AA cells, whose combined voltage will be 16V straight off the charger; there's nothing wrong with this as the voltage will quickly decline to under 15V.
 
I cant use a 12 volt regulator. I only have a 5 volt. I herd you can blow something going from 18-5 directly. Also, i dont have a proper relay, so a transistor may be my only option. Can i connect it so the 555 activates an npn tranisistor in which the npn activates the strobe? If so, can someone give me a shematic.?

Chris.
 
You should go shopping for the right parts. Finding a circuit that'll work fine with whatever you have on hand can be limiting.
Here's a link to how simple a set / reset relay would be.
relay-based SR flipflop
And it includes a simulator.
 
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I decided to skip the regulator/tranisistor idea cuz it wouldnt work correctly. I tried the relay idea since i know how to connect relays well.

I have an SPDT relay on my expermental board and it works fine with it. However, i am puting the cicuit only on a breadboard cuz the electonic kit has a short in it somewhere.

I went and bought a 12 VDC PC, Mini DPDT relay (i would have baught a 5 volt but 12 is all i could find at the stores).

It does not seem to work, even though i connected it the same as i connected it to the SPDT relay, but it never worked.

Can the DPDT relay work for what im trying to do? I dont see why not since the contacts are the same, only difference i see is that there are an extra set of NO, COM and NC contacts.

Does anybody know why it may not be working? I think the relay is not getting any power. I am using 2 6 volt batts in series, for 12 volts.

Chris.
 
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