A useful circuit snippet #2 - turn led on from ground!!!

bajaman · Post by **bajaman** » 14 May 2009, 22:49

Okay - maybe it is not a complete ready to build project, but some members may find it useful.
I had a problem recently where I needed to illuminate an LED indicator light when a relay was switched from +5v to ground potential( relay on state to relay off state) in an overdrive circuit built in to a vacuum tube guitar amplifier.
The solution for me was to use a very common CMOS IC - the CD4069, a hex inverting buffer

. This CMOS chip is extremely cheap - you should not have to pay more than US thirty cents for one at the most.
The CD4069 has six individual complementary N and P type mosfets, a ground (or negative) pin and a positive ( or ground) pin marked Vdd and Vss respectively (d = drain, s= source).
Okay - connect all the input pins together (1, 3, 5, 9,11,13) and connect all the output pins together (2, 4, 6, 8, 10, 12).
Paralleling the inputs and outputs like this creates a single inverter with six times the switching current capacity.
Now connect pin 14 to the positive voltage source (+5v to +15v dc), and pin 7 to ground.
Now connect a 1k5 resistor (for current limiting - can be higher for a dimmer LED light) to the cathode ( short lead on LED)to the bundled inverter output pins.
Connect the other end of this resistor to ground.
Okay - now when you connect a positive voltage to the bundled inverter input pins, the LED will not light up.
Once this positive voltage is removed or switched to ground, the inverter will reverse the state and output will be at whatever the supply voltage is being used on pin 14 - the LED will light up.

i know this is a very simple project but hopefully some FSB members may find it as useful as I have.
cheers
Steve

gus · Post by **gus** » 16 May 2009, 21:19

Why not a PNP transistor a LED and a resistor? E to 5V, B to the top of the coil and a LED and resistor to ground in the collector leg. When the Base and Emitter are at 5V the transistor will be off and when the base is at ground the transistor will be on, maybe add a series resistor in the base leg.

This is if I understood the post correctly.

bajaman · Post by **bajaman** » 16 May 2009, 21:28

[quote][Why not a PNP transistor a LED and a resistor/quote]
Yes, that will work as well.

bajaman

himister · Post by **himister** » 16 May 2009, 22:23

I've used this one.
Chose one of the LED's for different state of relay, and omit other transistor and two resistors along with LED if not needed.

Hope this would be helpful!

bajaman · Post by **bajaman** » 16 May 2009, 22:25

thanks himister

Just goes to show that the problem can be solved in a number of different ways.

bajaman

himister · Post by **himister** » 16 May 2009, 22:28

Note.

Resistors R6 and R8 should be 2k2 as R2, if 10V supply is used!

himister · Post by **himister** » 16 May 2009, 22:30

bajaman wrote:thanks himister
Just goes to show that the problem can be solved in a number of different ways.
bajaman

No problem!

That's why we are here!

himister · Post by **himister** » 16 May 2009, 23:03

Here it is! The quick pcb and layout!

In case anyone needs!

himister · Post by **himister** » 17 May 2009, 00:50

corrected

bajaman · Post by **bajaman** » 17 May 2009, 01:17

very clever - you should repost this as a completely separate topic (with a brief explanation of how it works

) in the Ready to Build projects section of the forum

Welcome to the forum

bajaman

himister · Post by **himister** » 17 May 2009, 02:33

bajaman wrote:very clever - you should repost this as a completely separate topic (with a brief explanation of how it works ) in the Ready to Build projects section of the forum
Welcome to the forum
bajaman

Thnx very much Bajaman.

I will post it along with other one I'm working on now, cause it's a part of it!

pz · Post by pz » 17 May 2009, 07:32

Baja

if i understand correctly, you are controlling the relay by applying the +V to the coil?
I don't know the exact circuit, but i think leaving an ultra high impedance CMOS inputs floating is just asking for potential problems. Things look much simpler if you control the realy from the negative side.

himister

i think your circuit is a little bit too complicated for such a function. It also draws the current from the source just to turn the LEDs off. It can be made much simpler, using only one transistor:

: relayctrl.gif (3.1 KiB) Viewed 1670 times

It also gives you an inverted logic signal to control other relays, switches etc.
I have used it a lot in my latest amp and it works great.

bajaman · Post by **bajaman** » 17 May 2009, 09:16

Hey - nice to hear from you again pz

That amplifier looks very complicated

The cmos inputs are not floating in my design - good point though

cheers
bajaman

pz · Post by pz » 17 May 2009, 10:21

The amp is a hybrid tube/solid state/digital homerecording guitar station. More info&phots are here, if you are interested. I had a lot of fun while building this monster

And here's more ideas using 4069 inverters:

bajaman · Post by **bajaman** » 17 May 2009, 10:29

The first picture is exactly what I used (minus the top LED and resistor)- the rectangle in parallel with the diode being the relay coil.
cheers
bajaman

himister · Post by **himister** » 17 May 2009, 15:07

pz wrote: himister
i think your circuit is a little bit too complicated for such a function. It also draws the current from the source just to turn the LEDs off. It can be made much simpler, using only one transistor:

Thnx man. Another way to do it! Great!
This one is drawn from the original circuit made in Laney preamps, but I assume there's a reason why works like that since it has a common PSU with FX loop circuit. I've just added one more tranny, LED and two resistors for extra function of relay logic state!

Anyway it's very useful. If anyone needs I'll draw PCB for your prototype gladly!

Cheers