help with a small switching circuit circuit - Page 1
Forum section: electronics / beginners

author: dak4482
author_rank: Newbie
timestamp: February 07, 2022, 05:08:25 am
content: HI all,I'm a very very new hobbyist and am trying to simulate a small circuit from the Apollo DSKY unit.Attached is the circuit as shown in the schematic. I want to be able to simulate in Proteus (thats the simulator I have) that if a logic 1 is applied "a logic 1 shuts off Q1, which holds Q2 off, and allows Q3 to conduct" which turns on "Relay Row B ON". I the assume if a logic zero is applied the Q2 turns on wgich turns on "Relay Row A On".The schematic is correct except for the transistors. No transistor types are given . The NPN and PNP designations are as shown in the schematic. The ground signal is actually zero volts DC in the Apollo diagram but I saw in a video that Proteus does not react well without a ground. If you need the 0 volts DC to accomplish the circuit please do so,If anyone can plug in the correct transistors for this small circuit to work It would be GREAT. A type number that I can select from a library would be VERY helpful.No matter what combination of transistors I try I cant get Q3 to turn on.I am 70 years old so this is obviously not a homework assignment haha. Just trying to understand the workings of the Apollo Dsky unit and maybe simulate small parts of it.Any help would be greatly appreciated.SEE ATTACHMENT FOR CIRCUIT DIAGRAMDan Kosko

author: ledtester
author_rank: Super Contributor
timestamp: February 07, 2022, 06:28:10 am
content: Quote from: dak4482 on February 07, 2022, 05:08:25 amCan you supply the schematic of the DSKY circuit?QuoteYour supply voltage is 13V which means your "LOGIC 1 or 0" signal will also need to be 13V for a logic 1. However, the voltages uses for logic signals in Proteus are probably either 5V or 3.3V.An input voltage of > 12V is needed to keep Q1 off, and if Q1 is on then Q2 is on and that will force Q3 off. So if the input never exceeds 12V then Q3 will always be off.

author: fourfathom
author_rank: Super Contributor
timestamp: February 07, 2022, 06:45:50 am
content: Also, when Q1 turns on it drives the base-emitter junction of Q2 with no current limiting. One of those transistors will fail. I suspect that R3 should connect between the Q1 emitter and R4. This will limit the current to more reasonable levels.

author: rstofer
author_rank: Super Contributor
timestamp: February 07, 2022, 06:45:07 pm
content: The base of Q1 is already pulled high - presumably logic '1'. All you need to do is pull the input down to ground to cause the relays to switch states. The appropriate resistor, R1, is already chosen.If you want to drive the circuit with an NPN transistor, connect the collector to IN, connect the emitter to ground and connect the base to your external logic level. Maybe put a 10k resistor in series with the base. You can also use a toggle switch between IN and Gnd to simulate whatever is happening outside for a real circuit. Maybe the simulator has a toggle switch that changes state at t=<something>.Mostly, I'm talking about a real circuit, I have no idea what the simulator is going to do. I suspect if you add the new NPN transistor and hit the base with a periodic square wave through a resistor, the relays will toggle. But I don't know that...

author: rstofer
author_rank: Super Contributor
timestamp: February 07, 2022, 06:51:31 pm
content: Quote from: fourfathom on February 07, 2022, 06:45:50 amYes, something weird (read as 'wrong') is going on with R3 and R4. One should be in series with the base of Q2.I certainly question the resistor values. I'm not saying they're wrong but they seem a bit low. I wonder what hFEis being considered and how much current the relays draw.That's a very nice drawing, I wish they all were this clean!

author: fourfathom
author_rank: Super Contributor
timestamp: February 07, 2022, 07:17:14 pm
content: Quote from: fourfathom on February 07, 2022, 06:45:50 amOops, I meant "between the Q1collectorand R4". And I agree, the R values seem generally too low, but that's a problem to be taken up after the basic topology is fixed.I also agree about the necessary logic level at the input. if it's really 5V or 3V you might as well eliminate Q1 and just drive Q2 directly through the properly-connected R3 base resistor (and swap the RELAY outputs, or the polarity of the drive signal). What's there now won't work unless the logic levels are +12V or 12V-tolerant open-collector.

author: dak4482
author_rank: Newbie
timestamp: February 07, 2022, 08:46:46 pm
content: OK,Heres the circuits with the input at both 0 and 1 logic. (SEE ATTACHMENTS)Still cant get Q5 to turn on. Had to change R6 to 10k and the source voltage to 5 volts to get it to work this far with Q4 turning on.. Any changes you'd suggest now?2022-02-07_15-38-16.jpg(126.96 kB. 880x514 - viewed 47 times.)2022-02-07_15-48-45.jpg(132.73 kB. 895x537 - viewed 51 times.)Still looking for those transistor types!!!!!!!Dan

author: dak4482
author_rank: Newbie
timestamp: February 07, 2022, 09:23:00 pm
content: FINALLY.Got it to work in the simulation. Had to add a 2k resistor R8 in the diagram. works fine now at least there. Here's the final schematic.Thx for the help.Dan

author: rstofer
author_rank: Super Contributor
timestamp: February 07, 2022, 10:22:03 pm
content: The problem noted above about R3 & R4 plus over-driving the base of Q4 hasn't been resolved. The simulator might accept it but I doubt a real transistor will.

author: ledtester
author_rank: Super Contributor
timestamp: February 07, 2022, 10:27:55 pm
content: Quote from: dak4482 on February 07, 2022, 05:08:25 amI'm still very curious about the original DSKY schematic. Can you post that?

author: dak4482
author_rank: Newbie
timestamp: February 07, 2022, 10:52:37 pm
content: ledtester,the original schematic involves hundreds of relays and tens of the small schematic I have shown to drive rows and columns of those relays. I'll try to attach it though I'm not sure how well it will come out as its pretty big. This part of the schematic selects only the rows of the relays that drive the INDIVIDUAL 7 segemnts of the displays. (second attachment Its pretty involved. Thats why I'm taking baby steps hahahDan

author: rstofer
author_rank: Super Contributor
timestamp: February 07, 2022, 11:31:46 pm
content: I would hesitate to call the original schematic wrong but the problem with R3 and R4 seems institutionalized.

author: dak4482
author_rank: Newbie
timestamp: February 08, 2022, 02:03:23 am
content: Well that very well could be. Here is the very latest version that is shown in the NASA archives that is a flight approved version. Quite a difference.. Any major flaws that you can see? Both the source voltages R3(1) and R5(1) are shown as 28VDC with O VDC at R1(2).Dan

author: fourfathom
author_rank: Super Contributor
timestamp: February 08, 2022, 02:56:13 am
content: R3 and R4 in the "original" complex schematic are still wrong.The latest one from the NASA archives (?) is essentially undecipherable, but from what I can tell it looks very wrong. How are the power and ground connected? I can tell you that Q3 probably needs a series base resistor if it is being driven at logic levels, and C1 seems to have the polarity reversed (if Q2 is actually an NPN).What do you want the circuit to do? What will be the input, the output loads. and the supply voltage?

author: ledtester
author_rank: Super Contributor
timestamp: February 08, 2022, 03:05:13 am
content: Quote from: fourfathom on February 08, 2022, 02:56:13 amHere is where I think the second circuit comes from:https://archive.org/details/acelectroniclmma00acel_0/page/n348/mode/1up?view=theaterA blown-up view:It appears Q1 in that schematic is a PNP transistor and Q2 an NPN. Q3 is probably also an NPN.