First Project: Voltage Indicator

Wendy

Joined Mar 24, 2008
23,415
The way I'd do it is the input and voltage divider would be the same circuit. A second voltage divider would be the reference voltages, and have a precision voltage regulator on it. Both these voltage dividers would feed into comparitors, one for each logic state (yes/no). The output of the comparitor would trigger the appropriate signal.

A circuit like this is very much like a flow chart in many ways.
 

tibbles

Joined Jun 27, 2008
249
as bill says much of the fun is finding out for yourself ,you are doing great. there are countles examples of the circuits you need on the internet, treat them as building blocks.
i usually draw a small waveform of the input and output to and from each chip
to get back to basics- your power supply, you will need at least 14 volts on your divider chain . you will need to check out your comparators supply voltage.and whether they are single supply.
good luck
 

tibbles

Joined Jun 27, 2008
249
sorry i missed the 555s these wiil run on anything from 5v to 18v
and can source or sink 200ma,, so they should run your output componants directly,with no buffer needed, i'll butt out now
good luck again
dougal
 

Wendy

Joined Mar 24, 2008
23,415
One hint for the flashing LED,

555 Hysteretic Oscillator

Neat thing about this oscillator, you can still use pins 2 & 6 as inputs, forcing a logic state and killing the oscillator. For example, if a LM339 comparitor were connected to pins 2 & 6, and it's output were low, the oscillator output would be forced to a high. Pin 4 of this same oscillator will force it to low, which covers all the states you need for the LED.

BTW, don't forget you can make funtional digital gates using diodes and resistors.
 
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Thread Starter

marx

Joined Oct 4, 2009
64
Awesome thanks Bill, Bit busy at the moment but once I've had some time to look into the ideas put forward, I'll start asking some more questions...... lots of questions ;)
 

Thread Starter

marx

Joined Oct 4, 2009
64
The design currently in my head was going to use the 555 as the LED driver, both for the flasher and the solid indicator. I'll wait for the OP to get back with us.
I've just been looking at the 555 and I'd have to admit I have no idea how to implement it but it does look like it will be suited for what you have suggested.

Are you thinking of using just the one LED for solid and flasher or separate LEDs.

I've roughly drawn up the Input Comparator Section with resister values

http://i906.photobucket.com/albums/ac267/marxses/InputComparator.jpg



For the power supply that sets the reference voltage, does that need to be regulated to provided an accurate reference? The 18V I have there is just from two 9V batteries in series. Would I use the same supply rail to power the comparator?
 

Thread Starter

marx

Joined Oct 4, 2009
64
The other question I wanted to ask was can you attenuate the input signal say by a factor of 3 and then do the same for the reference voltages? This way I would only need the one battery.
 

Wendy

Joined Mar 24, 2008
23,415
The other question I wanted to ask was can you attenuate the input signal say by a factor of 3 and then do the same for the reference voltages? This way I would only need the one battery.
That was exactly my thought when I said you could use any battery you choose. I'll get back with you on the 555.
 

Wendy

Joined Mar 24, 2008
23,415
OK, here is what I was talking about. A 555 in this configuration has two inputs. Pin 4 (Reset) brings the output low no matter what (it also resets the internal flip flop). When pins 2 and 6 are connected the chip becomes an inverting Schmitt Trigger, which gives us another input.

Betcha can figure out the truth table without too much heartburn.



I didn't show them, but plan on several bypass capacitors (0.1µF and 220µF).
 

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tibbles

Joined Jun 27, 2008
249
hi bill ,just spotted your circuit could be a useful one to remember,but truth tables are a bit over my head,-pass the rennies

a and b high =o/p low ?
a-Low and b-high= o/p low ?
a-low and b low = o/p high ?

somthing like an inverting and gate

d
 

Thread Starter

marx

Joined Oct 4, 2009
64
So C1 and R1 set the frequency of oscillation? What formula do you use to calculate this?

Whats the purpose of Q1?
 

SgtWookie

Joined Jul 17, 2007
22,230
So C1 and R1 set the frequency of oscillation?
Yes.
What formula do you use to calculate this?
Roughly, frequency = 0.7RC.
The first ON cycle will be somewhat longer due to the extra amount of time required to charge C1 to 1/3 Vcc.

Whats the purpose of Q1?
Q1 is used as a switch. If input B is high, Q1 conducts, which discharges C1.
If input B is low, C1 is charged and discharged from pin 3 via R1.
 

tibbles

Joined Jun 27, 2008
249
hi marx would you mind if i draw where youve got to,
i need the practice.... it would also help us with any questions
i like the idea of splitting your voltages but this might be a bit confusing especiallyif you need need to introduce a voltage reg
can you only use 1 led
iwould be tempted to go with 18 v but ive been wrong before
regards d
d
 

Thread Starter

marx

Joined Oct 4, 2009
64
Yeh thats fine tibbles, I'm trying to draw something up in Orcad which i have been using for all of one day now.

Bill I'm a little confused with the truth tables.

From LM339 to 555, I have the voltage above 14V as input A and the voltage below 10V as input B.

Do you make up a truth table for LM339 and the 555 i.e

LM333output

.LED
A B State
0 0 .Off
1 1 .Off
0 1 Flashing
1 0 .On

Then as you stated for 555 input

.LED
A B State
0 0 .Off
0 1 .Off
1 0 Flashing
1 1 .On

Do you add them together to find the appropriate logic gates at the comparators out puts?

Otherwise I don't see how both A and B can be 1 given that A is above 14V and B is below 10V
 

tibbles

Joined Jun 27, 2008
249
thanks marx, im hoping someone will correct, i,m learning myself,
the neg out put may seem odd, but 555s usually need a neg going trigger input,
 

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Thread Starter

marx

Joined Oct 4, 2009
64
I've got the outputs going high when the voltages are outside the 10-14V range. Figured that made more sense as we are designing this circuit to indicate voltages outside this range.
 
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