How to drop ~1.6 volts across a single 1206/SMD space? ATtiny85/Arduino SOLVED - Page 1
Forum section: electronics / beginners

author: t1d
author_rank: Super Contributor
timestamp: December 06, 2021, 09:00:23 am
content: BackgroundMy project partner and I are using an ATtiny85/Arduino IDE to CharliePlex blink patterns on a Christmas Ornament. He is handling the coding and I am handling the hardware. He is used to working with ATtiny85s/Arduino IDE and I am used to working with PIC12F683s/MPLab X IDE. The project is non-commercial = personal decorations and gifts for family and friends.We want to use the RESET pin for the menu button to select different patterns. The menu selection is advanced, when the pin is pulled low, through a switch to ground.The ProblemWe had a communication issue, regarding the needed voltage to hold (pull up) the pin high. I now understand that the needed voltage is 4 volts, not the full 5+ volts of the supply. This is normally accomplished with a simple resistor voltage divider.As I originally thought that the pin was held high at the full 5+ volts of the supply, I only provided component space for a single, current limiting resistor. The PCB boards (30) have already been manufactured, Because the board design uses high density SMDs, there is not any room to bodge in a second resistor to make a voltage divider. Meaning, we need to drop the voltage across a single 1206/SMD space.Solution IdeasChanging the resistor out for a diode comes to mind, but that would only drop 0.6-0.7 volts and I don't think that is not enough of a reduction(?). Using a LED would drop more voltage, but there is still no room for its current limiting resistor and it would glow continuously. I am not aware of an incandescent bulb in a 1206 package. As the voltage is DC, I don’t think a capacitive dropper would work.RequestObviously, we need a suggestion for a solution, be it hardware, or code. As said, I do not know much about the Tiny, or the Arduino IDE. But, I recall that my project partner said that we do not want to “set fuses” and we do not have a high voltage programmer. Our programmer is an USB dongle.I have done my best to state things clearly and to use the proper terminology. But, please don’t let that fool you; I am just a hobbyist, as to circuit design and coding. Kindly share your ideas with great detail and instruction.Thank you for your help and support.

author: srb1954
author_rank: Super Contributor
timestamp: December 06, 2021, 09:42:23 am
content: Quote from: t1d on December 06, 2021, 09:00:23 amAre you referring to the logic high voltage required on the RESET pin?If so, it is perfectly fine to simply have a pull-up resistor to the +5V supply. The 4V you are referring to is probably just theminimumvoltage that the pin has to be pulled up to for it to be recognised as a valid RESET logic level.

author: DavidAlfa
author_rank: Super Contributor
timestamp: December 06, 2021, 10:15:01 am
content: Why do you think you need 4V? 5V is totally fine. The circuit looks ok to me.I didn't understand the issue, please explain it better. Is PB5 connected to anything else?By default RESET input is enabled, perhabs you forgot to disable it by setting RSTDISBL it to 0 in the fuse bits?Otherwise, pushing that button will reset the mcu.If the button polarity was reversed, it should be easily fixed in software.

author: t1d
author_rank: Super Contributor
timestamp: December 06, 2021, 10:28:03 am
content: Thanks, folks, for your efforts to reply. Good to know that the 4V is the minimum needed voltage and that the circuit is okay, as is.But, if the resistor divider had been truly needed, I clever solution did come to me... The momentary switch is a SMD/2 Pin/Gull Wing-Like. It is about the same length as a typical 4 Pin Mom Tact Switch, but it is only half as wide. There is room to abut a Through-Hole resistor to its long side. Then, bend the resistor's pins around to the switch pins and solder them. This would create the resistor path for the resistor in the divider that goes to ground, because the switch goes to ground. Press the switch and the resistor divider would be grounded from its midpoint.Thanks for your excellent help. It is very much appreciated!

author: DavidAlfa
author_rank: Super Contributor
timestamp: December 06, 2021, 10:29:54 am
content: Yep, easy fix, there're plenty of places to do so, ex. between the attiny pins 1-4.But how you came to the conclusion you needed 4V in first place?

author: t1d
author_rank: Super Contributor
timestamp: December 06, 2021, 10:36:29 am
content: Quote from: DavidAlfa on December 06, 2021, 10:29:54 amMy project partner supplied me with the chip details. He was having trouble with the functionality of the menu select, via a button press, and that is what made him research the issue. He must have missed the detail that the voltage is a minimum, or got bad internet info. There's a lot of that out there.

author: DavidAlfa
author_rank: Super Contributor
timestamp: December 06, 2021, 10:46:42 am
content: So it all was a technical missunderstanding, not an actual issue?

author: Steffalompen
author_rank: Contributor
timestamp: December 06, 2021, 05:59:56 pm
content: On a side note, isn't your supply voltage possibly a smidgeon high with fresh 2032s?Worst case 2*3.25V - 0.4V = 6.1V, (possibly 3.4V?)AtTiny85 max 5.5V Absolute 6V