First Project: Voltage Indicator

Wendy

Joined Mar 24, 2008
23,415
R14 and R15 are 100KΩ? Seems to me that would starve the devices for current. I missed it too. You would be better putting a emitter follower to use as a voltage regulator similar to what I did, and dropping the resistor values. As is, you can only get 0.1ma through that, and it gets worse when loaded.

Look at a functional diagram of a 555, you'll note it has 3 5KΩ resistors in series internally, not counting the rest of the circuitry. The CMOS 555 will be much more efficient though, but you're also trying to light LEDs (10ma) with that too.

 

Thread Starter

marx

Joined Oct 4, 2009
64
Thanks Bill I picked that up so I've done another revision.

I decided to make the supply rail 9V and attenuate the input signal. The power supply I have isn't very accurate. The stated 9V is actually 9.15 so that why I've got a few strange values.

Man its hard keeping track of all the different values.

 

bertus

Joined Apr 5, 2008
22,270
Hello,

I have followed the thread for a while.
For more stability at the voltage reference you could use a TL431.
See attached datasheet.
This is an adjustable voltage reference, that can be used like a zener diode.
This could be connected to pin 4 of the 339.
To have regulation the resistor between +9 Volts and pin 4 of the 339 should be smaller.

Greetings,
Bertus
 

Attachments

Wendy

Joined Mar 24, 2008
23,415
Problem is, he has a standard list of materials (see post #20), and that isn't on them. They did allow for Zeners of any value however, which is why I went the route I did.

To Marx, it is a lot simplier to parallel resistors than to put them in series. That string of three, for example, could be two. I suspect your instructor won't have any problem with you declaring the second larger resistors are test selectable. The math to calculate parallel resistance is pretty easy, I use a hand held calculator to figure them.

Is it me, or is your design shrinking? :)
 

Thread Starter

marx

Joined Oct 4, 2009
64
Thanks Bill taken note on the parallel resistors will have a look at it when I do my next project as I've just about finished making this circuit.

I'm not using a breadboard this time round. Just a wooden plank that I've drilled holes into and wired around it. Just couldn't visualize the circuit on the breadboard I guess that will come with experience.

Yeh I think I've managed to simplify the circuit a little from my original design. Have to say that this has been a heap of fun and I'm really appreciative of all the advice so far.
 

Wendy

Joined Mar 24, 2008
23,415
Engineers and techs find fun in other than normal places. I enjoyed making my design, it is a little like a puzzle.
 

tibbles

Joined Jun 27, 2008
249
Bill ,perhaps you could put one of your breadboard pics up, so we can see how it should be done,

i can never get the damn things in the holes...
 

Thread Starter

marx

Joined Oct 4, 2009
64
I took out the voltage dividers in front of the LED as they won't work on the stated voltage of 2V.

So now without any input voltage the buzzer and LED go when they shouldn't...... any ideas whats wrong.
 

Wendy

Joined Mar 24, 2008
23,415
You're using the transistor as part of the current path. Remove R1, R5, R16, R25, and R26. Put a current limiting resistor to ground on the cathode of LED2. The transistor (with the buzzer and resistor) is a high impedance input.
 

ELECTRONERD

Joined May 26, 2009
1,147
The data sheet I have says max 2.3V for the RED LED at 20mA. It works well at 9V 15mA
Then is that what you'd like, 15mA? It isn't that difficult to get the proper resistor value according to that 20mA current. I'm assuming when you say "2.3V," that represents the forward voltage drop. Thus:

\(9V-2.3V=6.7V\)

\(\frac{6.7V}{20mA}=335 Ohms\)
 

Thread Starter

marx

Joined Oct 4, 2009
64
Okay so I mistook the forward voltage drop for max voltage rating.

I've taken those resistors out but the LED and Buzzer go regardless of the input signal........ for some reason the transitors are turning on when they shouldn't be.
 

Thread Starter

marx

Joined Oct 4, 2009
64
This circuit won't work. The output of the 555 is set to pulse at 1hz, this would cause the LED to flash at the desired 1Hz but also cause the transistor switch the buzzer to go on and off at 1Hz and this is not desired at all.

Have to rethink the whole thing and its due tomorrow morning, with theory and photos :(

Think I'm going to have to use elements of Bills circuit.
 

Thread Starter

marx

Joined Oct 4, 2009
64
Bill another question about this circuit. With the supply rail connected to the output of the LM339 wouldn't the circuit draw power from the supply rail regardless of the comparator being high or low?

I'm struggling to come up with the appropriate resistor value for my circuit mainly in regards to fully saturating the transistors..... I'm not even sure if the LM339 sink voltage/current is enough to saturate them. The transistors I have are BC558s.

Actually I'll put the names of the components in my schematic now
Bill has the Discharge Pin (7) deliberately been left out. Does it need to be grounded?
 

Thread Starter

marx

Joined Oct 4, 2009
64
1.30 in the morning and I still can't get it to work........ tried everything..... just removed LM339 to see if faulty.

Should both output 3 and 4 measure the same voltages with respect to circuit ground when they are in there low state.
 

tibbles

Joined Jun 27, 2008
249
hi marx
been off or the last week or so, but judging by your last post you were struggling to meet the deadline,
thinking about it, it dose'nt really matter,you could have gone on the internet in week 1, pulled out a nice shiny circuit. put it together in a couple of days-- job done.. and learnt nothing.
instead of which you have worked through every detail of the project, your drawings have improved a million per cent,you now know as much about comparators, dividers, transistors, etc. as many of us here.

good luck

dougal
 

Thread Starter

marx

Joined Oct 4, 2009
64
Got it finished to some degree..... worked all that night trying to trouble shoot why the circuit wasn't working only to find at 8am that the 555 timer I was using was broken :mad: Is there an easy way to test IC components?

Also wasted a heap of time not using a breadboard which Is what I ended up using... not sure why I couldn't work them out initially as they are pretty basic and save a heap of time.

So got it working but didn't get time to write up the theory, If I get 50% I'll be happy.

Time to study for the exam.
 
Top