I am working on a very simple, relatively low power, signaling system for an oral surgery office.
There are 4 suregery/exam rooms.
-Each will have a panel w/ 2 SPDT illuminated latching pushbotton switches (one red and one green)
There are 5 signal panels (areas where nurses might be)
-each panel will have 8 illuminating indicators ( 4 red and 4 green, corresponding to the 2 switches in each of the 4 rooms)
I am using a 74HC595 8 bit register to illuminate the appropriate indicators when the corresponding buttons are pressed.
The signals from the switches are brought into the arduino via a 74HC165N Parallel in/ serial out register.
the switches and indicators are all from NKK switches
The illumination for the red switches / indicators is: 1.9V, 20ma typ., 30ma peak
The illumination for the green switches / indicators is: 2.1V, 20ma typ., 30ma peak
I have mocked up a proof of concept / small scale prototype w/ 4 switches and 4 indicators (2 red and 2 green of each)
Problem:
This one might be very simple and just be a matter of the resistors but im not entirely sure.
The 4 switches and their corresponding indiciators are each illuminated by 1 of the chanels of the 595.
The 595 is being powered by the UNO's 5v lead.
The circuit goes from the output pin for the individual chanel on the 595, to a single 220ohm resistor, to 2 leds in parallel, to the arduino ground.
The red switches and indicators have the exact same intensity of each other and the exact same intensity on both chanels.
The problem is with the green switches/indicators.
On the first chanel the green switch is just as bright as the reds while the green indicator is aprox 1/2 as bright as the green switch.
the second green chanel is the exact opposite. The indicator is jsut as bright as the reds and as bright as the green switch on the first chanel but the switch is 1/2 as bright.
Any ideas?
I am assuming that it has something to do with the fact that the greens require more voltage and the fact that they are using the same value resistor as the reds. But I would also assume that the 2 leds would be equally bright (just not as bright as the reds).
I know this is a lot to take in but I usually find that problems are easier to fix when you know the whole story.
Thanks in advance for any help.
The circuit goes from the output pin for the individual chanel on the 595, to a single 220ohm resistor, to 2 leds in parallel, to the arduino ground.
What's the current being drawn? You said the LEDs need 20ma typical and your driving two in parallel, but if you look at the 595 specs
±6-mA Output Drive at 5 V
The 595 is not intended to be used to directly drive LEDs, OK for mucking around on a bread board but you should really use a higher current device like the TPIC6B595. Same pinout and functionality but high-current drive.
Thank you everyone for your help. I did not look closely enough at the specs of the components I was using.
I currently have the circuit set up using the 74HC595 from NXP
I also have 2 of the TPIC6B595N's that I can switch to that I bought for messing around with.
I will look more into the specs of each chip and make a better informed decision.
Magician:
to a single 220ohm resistor, to 2 leds in parallel, to the arduino ground.
You can't connect leds in parallel, and expect equal current/brightness.
This ^^^^ was not something I was aware of. I have made LED arrays before with just a power supply, relay, and resistors and I end up with something like 10 series (containing 3 led's and a resistor) in parallel and the different series always have the same intensity (unless I have a bad led). Is it because the two parallel leds are being run off of the same resistor?
This is just a mock up to prove it will work. In the final version each LED will have its own resistor.
It took me 5 min to find an answer, even I'd known what to look for. And you know what? This doc was on 5-th line, but instrauctables show up as number ONE!
Unbelievable
