Very green and need help (RE: EEVBlog #262) - Page 1
Forum section: electronics / beginners

author: nukejr
author_rank: Newbie
timestamp: November 16, 2012, 02:57:47 am
content: I saw Dave's YouTube Video on EEVBlog #262, Soft Latch circuit. This is exactly the circuit that I need!! (http://www.eevblog.com/2012/03/30/eevblog-262-worlds-simplest-soft-latching-power-switch-circuit/)History: I have a pickup truck that has a video screen in the rearview mirror and is designed for safely backing up or connecting to a trailer. This video screen currently is only activated when the truck is shifted into reverse. However, I would like to install a switch that will allow this screen to be turned on regardless if the truck is in reverse or not. This allow me to periodically monitor my trailer while in tow.Current Situation: I went and purchased a switch from Toyota that is exactly like other switches in my truck, and is a momentary pushbutton switch. This will keep it looking like it was a Toyota designed feature.Goal: Build this circuit to integrate with a relay to allow bypassing the "Reverse Gear" permissive signal and turn on the video screen at anytime, but not lose the factory installed function to have the screen come on when the truck in placed in to the Reverse gear.Problem: My truck is all 12VDC power and Dave's YouTube video assumes a 3V systemI have the dexterity and determination to successfully build this circuit, but I lack the technical knowledge to determine which specific components I need. Below is the circuit diagram. All values are just what the software defaulted to are not accurate as far a I know. But those values or specific component number are exactly what I need to know.For clarification: SW2 is the "Reverse Gear" switch and is closed only when the truck is placed in the Reverse GearSW1 is the switch I want to install.D1 symbolizes the screen activation signal.Thanks in advance for any help!

author: IanB
author_rank: Super Contributor
timestamp: November 16, 2012, 03:11:13 am
content: I guess I'm a bit puzzled. Why is this arrangement of no use to you?(The momentary switch will activate the rear view camera when you press it and turn off when you let go. Isn't that enough for a quick glance to check everything's OK?)

author: nukejr
author_rank: Newbie
timestamp: November 16, 2012, 03:23:20 am
content: I would like to have the option to keep the screen on. Otherwise, Yes, for a quick peek your circuit would work flawlessly!!!

author: IanB
author_rank: Super Contributor
timestamp: November 16, 2012, 03:32:23 am
content: At a rough guess Dave's circuit will work fine at 12 V. Since it's a switch circuit it won't really care about the supply voltage as long as it's lower than the maximum rating of the components used. I don't think 12 V is high enough to be a worry for most jelly bean items.A deeper issue is the automotive electrical supply. Such systems are notorious for noise and spikes and other nasty stuff that can blow things up. To protect the circuit from failure you would probably need to add some good filtering and smoothing and voltage regulation on the power supply side.Is there no way you can use a toggle switch instead of a momentary push switch? It would be much simpler and more reliable.

author: nukejr
author_rank: Newbie
timestamp: November 16, 2012, 07:02:43 am
content: If you think the circuit will work, then I'll buy the components as described in the video and give it a shot!I'll also look into integrating a filter/regulating circuit. I'll be amazed if I can get it to work!As fas as the toggle switch, I don't have that option because I want it to look OEM. The switch I bought from Toyota is exactly what I already have in the truck, and there are no toggle switches in the truck, with the exception of maybe the window lockout switch.Besides, using the momentary switch and having to build this circuit adds a level of complexity and creates a challenge that I look forward to completing.Just in case you are interested, I'll post pictures and/or a video of the results.

author: sporadic
author_rank: Regular Contributor
timestamp: November 16, 2012, 01:22:30 pm
content: You doing this in a Tacoma? If so, have a part number for the switch? I'd be interested in doing the same for mine. Thanks!

author: nukejr
author_rank: Newbie
timestamp: November 28, 2012, 01:20:51 am
content: I struggling here. I have assembled the circuit, as shown below, and it is not working as I had hoped.The result of my efforts has created two things: (1) An acceptance of how limited my intellect really is, and (2) A circuit, that when energized, immediately lights the LED and upon maintaining the switch closed, causes the LED to then oscillate on and off. Momentary closure only results in the very brief initiation of the oscillations. Upon opening the switch, the LED is constantly lit.If anyone is familiar with the circuit created by Dave on the above aforementioned video (#262), he used a P-channel MOSFET. However, I am unable to locate and purchase the MOSFET as described in the video, hence the use of the transistor in the diagram below. If using the MOSFET is the only way to go, and anyone knows of an alternative to the IRF9110, I'd appreciate the info.Otherwise, can someone diagnose my problem (in regards to the circuit!) or suggest/direct how to correct the circuit, possibly using the parts shown?As a side note, I am reminded of a proverb my grandfather (a pioneer in the computer science field) once told me early on in my PASCAL programming days: A program will never do what you want it to do, only what you write it to do. I have found that the same is true for circuit building!!sporadic, this will go into my '10 Tundra. The switch part number is 847200C010.

author: IanB
author_rank: Super Contributor
timestamp: November 28, 2012, 02:43:00 am
content: Quote from: nukejr on November 28, 2012, 01:20:51 amIt strikes me that your circuit is half working.The expected function is that momentary closing of the switch causes the LED to light up. A second momentary closure should cause the LED to go out. Therefore if you hold the switch closed you might expect the LED to go on, then off, then on, then off and continue in like manner. So you are close to a working circuit. What is not happening, if I read correctly, is any way to have the LED stay off when you let go of the switch?What I think might fix the circuit (only a guess) is to replace the 100 k resistor on the base of the 2N3906 with a 10 k resistor, and then place a 100 k resistor between the base of the 2N3906 and the battery + side. What this should do is force the 2N3906 transistor to be switched off and remain off until turned on by pressing the switch.

author: nukejr
author_rank: Newbie
timestamp: December 14, 2012, 02:52:17 pm
content: Well, in spite of my lack of knowledge, I got the circuit to work just as it was described in the video. I was not successful in using a 3906 resistor, but was able to find a P-channel MOSFET resistor and the circuit works like a champ (as shown in the top schematic).However, the circuit is still not exactly as I need. Here's the problem: When I first introduce power to the circuit, the circuit will immediately latch. As I describe in the opening post, this circuit will be used in my truck. When the truck is turned on, power will then be supplied to the circuit. However, I do not want the circuit to latch until I initiate it.Can anyone suggest a solution to have the circuit initially be in an unlatched state upon introduction of power?

author: G7PSK
author_rank: Super Contributor
timestamp: December 14, 2012, 04:12:10 pm
content: Just take the feed from a non switched supply rather than the ignition switch, most vehicles have one or two such lines like the interior lights cigar lighter socket radio etc.

author: nukejr
author_rank: Newbie
timestamp: December 14, 2012, 10:38:12 pm
content: I'd rather not connect the circuit to a constantly on power source. There has to be a cleaner (correct) way to do this. Anyone else have ideas?

author: IanB
author_rank: Super Contributor
timestamp: December 15, 2012, 03:41:21 am
content: Quote from: nukejr on December 14, 2012, 02:52:17 pmThat's exactly what Dave's circuit does in the video at time 12:07. So I don't think there is anything wrong with how you have constructed the circuit, I think that is how it behaves.I'd have to say that seems like a flaw in the the circuit, as most people would want a soft switch to start up in the off state.There may be a modification to the circuit that fixes that, but I don't have one immediately to hand. It would need a bit of thought and experiment to test it out. Sorry.

author: IvoS
author_rank: Frequent Contributor
timestamp: December 15, 2012, 07:12:18 pm
content: This should work.

author: cthree
author_rank: Frequent Contributor
timestamp: March 16, 2014, 01:00:39 am
content: Quote from: Electroalek on December 15, 2012, 09:32:54 pmBrilliant!! It works like a charm.I used a 0.1µF ceramic cap for C which turned out to be prefect. It's running at 5V and I am using it to drive the coil of a 5A relay. Hardly any components.