Question about basic 555 circuit error - Page 1
Forum section: electronics / projects

author: EncomLab
author_rank: Contributor
timestamp: December 20, 2016, 05:01:54 pm
content: Amateur here with a question about a 555 circuit that is not providing the output I anticipated. I attached a schematic from eagle to help. When I operate the circuit LED 2 illuminates, then shuts off and LED 1 comes on - which is as intended - however LED 1 is many times brighter than LED 2 which I did not intend. I simulated the circuit in Autodesk Circuits and it measures LED 1 peaking at 2.92V and LED 2 at 2.79 V - however on the physical circuit I am reading a peak of LED 1 at 2.55V and LED 2 at 2.15V. Did I create a voltage divider between the LED's? What should I do differently to have both LED's illuminate to the same level?

author: Audioguru
author_rank: Super Contributor
timestamp: December 20, 2016, 06:08:49 pm
content: Your battery voltage is not shown on your schematic! An Energizer CR1220 tiny battery produces 3.0V when its load current is much less than your circuit.The datasheet for the LM555 shows that its minimum supply is 4.5V and its output high voltage with a load of 10mA is the supply voltage minus 1.4V but its output low at 10mA is only 0.1V.Maybe you have or should be using a Cmos 555 which is an LMC555? Its minimum supply is 1.5V but its maximum output current is less than an ordinary 555 and is also not symmetrical.If your supply voltage is at least 4.5V then the 100 ohm resistors in series with each LED can be adjusted so that the LED currents and therefore their brightnesses are the same.

author: Ian.M
author_rank: Super Contributor
timestamp: December 20, 2016, 06:17:00 pm
content: Don't assume that equal LED currents will give equal brightness as their efficiencies may differ considerably. Consult datasheets for the specific LEDs you have used or if using generic parts with no known datasheet, set up a test jig to let you adjust the current through the LEDs side by side so you can compare their intensities, then use the actual measured current and Vf to let you calculate the series resistor to use for each LED.

author: Audioguru
author_rank: Super Contributor
timestamp: December 20, 2016, 06:25:12 pm
content: Ian,Do you know that at these low currents the output voltage of an ordinary 555 is not rail-to-rail but has a much higher voltage drop (1.4V) when it is high than when it is low (0.1V)? it is not symmetrical.

author: EncomLab
author_rank: Contributor
timestamp: December 20, 2016, 08:42:10 pm
content: Thanks for the explanation Audioguru - I am using a LM555CNFS-ND IC. I did not think about checking the datasheets as A. I am very new to electronics and B. the circuit is from Jeremy Blum and I assumed he knew what he was doing since he has a TON of tutorials and has authored several books on Arduino projects. I pulled the datasheet and the supply voltage min is 4.5 so obviously I am below that with the cr 1220. On the plus side I ordered my first PCB using this schematic so even though they don't really work I at least know how to set up a board, generate Gerber files and get a finished product

author: Ian.M
author_rank: Super Contributor
timestamp: December 20, 2016, 09:01:08 pm
content: Quote from: Audioguru on December 20, 2016, 06:25:12 pmYes, but as the NE555 is not rated for 3V operation, and you have already recommended a CMOS 555 to the O.P, which would be very close to symmetrical, I didn't feel the need to repeat the good advice you had already provided.

author: Zero999
author_rank: Super Contributor
timestamp: December 20, 2016, 11:43:34 pm
content: Quote from: Ian.M on December 20, 2016, 09:01:08 pmThe CMOS LMC555 does not have a symmetrical output stage. It is only specified to source 10mA but it can sink 50mA. Note that this is at 12V, at 3V the output current will be lower.www.ti.com/lit/ds/symlink/lmc555.pdfThis is fairly typical for CMOS devices. It's because N-channel MOSFETs have a lower on resistance than P-channel MOSFETs and lower supply voltages mean less gate drive. In CMOS devices with a near symmetrical output impedance, such as the 74HC series, the P-channel MOSFETs are considerably larger than the N-channel.

author: Ian.M
author_rank: Super Contributor
timestamp: December 21, 2016, 12:14:53 am
content: I thought it was closer to symmetrical than that. Certainly CMOS 555s when lightly loaded have rail-to-rail outputs unlike the bipolar version. However, it looks like it may not be possible to source the 4 to 5 mA current that would be required at such a low supply voltage even with the CMOS version. Increasing R1 to dim LED1 to match LED2 may be the only option.Also I mistakenly believed the O.P. had two different colour LEDs that would require intensity matching. If he's using named brand LEDs of the same type from a reputable supplier, purchased at the same time, the odds are fairly high that they will be reasonably well matched, however if he's sourced generic LEDs from Ebay or a discount grab-bag, it would be worth checking how well matched they are.

author: EncomLab
author_rank: Contributor
timestamp: December 21, 2016, 01:05:31 am
content: The LED's are the same color and purchased from Digi-key - C503B-GCN-CY0C0791-ND 5mm through hole green clear lens

author: Ian.M
author_rank: Super Contributor
timestamp: December 21, 2016, 01:19:23 am
content: So the odds are they are pretty well matched. If you cant increase the supply voltage, drop in a CMOS 555 as Audioguru recommended then increase R1 to dim LED1 to match the intensities. If you can increase the supply voltage to 5V or greater, recalculate the resistors allowing for the difference between the NE555 Voh and Vol drops.

author: westfw
author_rank: Super Contributor
timestamp: December 22, 2016, 05:16:32 am
content: QuoteWhile the current capabilities are not symetric for most CMOS equivalents of the 555, I believe the output voltages are pretty much rail-to-rail, so the ~10mA LEDs here shouldn't be a problem (that's a "10mA while maintaining specified output voltage" spec...)(Besides, the big problem here was that the supply voltage doesn't meet spec...)All things considered, putting in a CMOS 555 instead of the bipolar version should result in a big improvement.