need advice/comments on circuit

Hi, I'm new to electronics in general but I've installed a couple of headunits in my car, so I'm not completely clueless :) Just wanted to toss a circuit I'm planning to build out there so that people with proper electronics experience can have a look at it and let me know if I'm barking up the wrong tree, etc... Would appreciate any feedback, etc.. Have also included a diagram of the circuit. 4 different systems to be controlled by a single circuit, with 3 different modes(Normal, Rear Mode, Front Mode). 4 systems are: 1) Rear Parking sensors 2) Front parking sensors 3) Rear camera 4) Front camera Inputs to circuit a) Car reverse signal(triggered when car is put into reverse gear), 12v signal b) Front camera output(signal coming from front camera), RCA composite c) Rear Camera output(signal coming from rear camera), RCA composite Outputs of circuit d) Reversing sensors power(12v) e) Front sensors power(12v) f) Headunit Reverse signal(12v) - triggers headunit to display whatever is coming from camera input. If not on, headunit will NOT display camera input g) Headunit camera input. RCA composite Requirements: Normal Mode      - Front camera OFF      - Front sensors OFF      - IF car is put into reverse,           - Rear camera ON           - Rear sensors ON           - All front(camera and sensors) OFF Front Mode      - Front camera ON      - Front sensors ON      - IF car is put into reverse,           - Rear sensors ON           - Front Camera still ON           - Front Sensors still ON Rear Mode      - All front(camera and sensors) OFF      - Rear camera ON      - Rear sensors ON Assumptions: 1) Always have power supplied to cameras, and only switch the actual camera output via relay, so that display of image on HU is not dependant on startup time of camera 2) Where it says POWER on diagram, its switched power(ie. only powered when key is in ignition) so it does not drain the battery when car is not in use 3) When in normal mode, I want the default camera signal to be coming from Rear Camera. This is because in the headunit, there is an option to view the camera output WITHOUT the "Reverse HU Input" being triggered, and I want the default input to be coming from the rear camera Notes on proposed circuit: 1) First diode connected to "Car Reverse Signal" is there so that power from the switch does not feed back into where I'm getting the "Car Reverse Signal" from, eg. car computer so as to lessen chances I fry the computer 2) 3 position switch in circuit shows 4 possible selections as I couldn't find a better shape in Visio! middle two selections are joined in diagram to show they are both connected to same thing...eg. nothing! 3) Even though the POWER shape has an AC wave in it, its meant to be DC 12v. 4) Inputs into circuit are colored blue, and outputs are colored red. 5) Not shown on diagram but I'll be putting quenching/suppression diodes across the coils of the relays 6) I was initially thinking of just using the video switching relay to just switch the video signal, but after a bit more research, I'm thinking I should switch the grounds from each signal as well. I'll be replacing that SPDT relay with a DPDT relay(or just add another SPDT relay to switch the ground).

Hi Ween Yup, I already added a diode on the front mode switch output to the reverse HU input, its right there right before the junction where I connect the front mode output to the reverse HU input. Isn't that it, or am I missing something here? Also sorted with the mirror image with the rear camera and normal image with the front one. Anticipated that one so already accounted for it. current draw of the parking sensors are rated at 50ma, according to the specs. I ended up using relays mainly because I understand how they work, or think I do anyway... :) If you can suggest another option, I'll be happy to try that out as well.

Greetings Ghosts & Transcendentals, Ween means to stop +12V going from the reverse HU input to Front Mode position of the 3-way switch. Hence a diode in the Front Mode output of the 3 pos switch (pointing to the RHS - same direction as the reverse HU input diode. The same should/could be be done to the Rear Mode of the 3-pos to prevent the Car Reverse Signal feeding thru to the 3-pos supply. Hence 2 extra diodes if feed-back or -thru to the supplies is an issue. (Probably not an issue unless the Car Reverse Signal is NOT a typical mechanically switched +12V source - ie, from the gearbox or selector.) Quenching/suppression diodes across the coils of the relays may not be necessary, but better to be safe, and prevent interference to audio & video. One diode could do both Front Mode relays, but who cares - one per relay avoids any e-mag transmission etc. The books will probably tell you to use a matched 75-Ohm impedance (or whatever) coaxial relay for the composite signals, but having seen 75 & 300 Ohm antennae signal get switched with plain switches and relays, I wouldn't worry (and they are several hundred MHz and much weaker signals than ~1Vp-p ~6MHz composite signals). You may not need to switch the coax shield if it is grounded, but a DPDT relay would be ideal. (There are solid-state switches (integrated circuits) - but I'd try relays first. Far more robust, and simpler etc.)    You may have a synch-glitch when jumping from one camera to the other, but maybe these days that will be corrected on the next frame...    I congratulate you on your design and especially your description! (If only other questions were so well defined.)    And FYI - I actually prefer the AC source symbol; to me it is obviously a power source. Whether a rectified alternator or battery is irrelevant (provided we know it means DC).

Well spark and Ween...This "jargon" is right up your alley...ey...?! ...I mean that respectfully ...Of Course...!  What is your background in electronics(old spark)...Sorry to interupt the post...I just noticed in your profile...There is no mention of it...So curiousity has gotten the best of me...And had to ask...Sorry OP...! And Ween...If i may ask...Your backround also...?M.E.C.P & First-Class

Go slow and drink lots of water...Procrastinators' Unite...Tomorrow!

Sorry above - I missed Ween's 2 replies; hence it seems out of sequence. And Tommy - how dare you! Have you no sensitivity to peoples' private lives?! Hmmm - not too sure what to put.... Jack of all, Master of many? I had my education interrupted by a 4 year injuneerink degree. (Started as electronics but got cheesed off & I majored in electrical.)    I had an interest in electronics etc and - because I didn't know any better - I often did the impossible. (Unfortunately that habit continued even after my degree.) I've been involved in various fields & industries including automotive, military, legal, social - but now tend to be somewhat anti-social. My last employment ended several years ago - a telecomms company - varied jobs from security, OH&S, systems management, power, product exits. Since then I've probably spent most time trying to bring automotive & political) hobbyists into the new millennium.

Aha, thanks for the feedback, ween and sparky :) All makes sense now, was really confused when you first mentioned the missing diode. I've redone the circuit diagram now to fit the extra diodes(extra logic and quenching ones), and also made the video switcher a DPDT relay. As per your suggestions, I've gotten rid of the SPST relay leading to the "Front Sensor Power" and replaced it with a straight power from the switch. I've kept the other SPST relay that leads to the "Rear Sensor Power", mainly because I don't want to overtax the output coming out of the "Car Reverse Signal" as that is coming out of a CAN-BUS adapter, and in the documentation for that adapter, it states that this should be used as a trigger and not used to power anything. I think technically, it can be used as the sensors don't draw that much power, but better to be safe than sorry. With the diodes, based on my limited understanding of them, I can just use any 1N400X diodes right? I can get a pack of 1N4004 diodes quite easily, so I'm thinking of using them for all the diodes needed in this diagram. And thanks for the kind words, spark. It is very much appreciated and makes me think that I may just understand this yet!

Hmmmm, forgot to ask my relay questions... 1) Trying to figure out what a good relay to use for the DPDT video switcher relay. Trying to keep this as simple and inexpensive as possible, so like you said, discounting the coaxial relay for now, and just looking at normal run-of-the-mill relays. Looking at this post on something similar: http://www.installercentral.com/tech-tips/2009/07/10/making-a-video-switcher-using-relays/ .The diagrams from that post suggest that a 12VDC/3A DPDT relay from RadioShack(model 275-206) will be sufficient for this purpose, so I'm thinking that any relay that will do 3A and above will be ok for this purpose. I may be wrong with this so please correct me if I'm wrong. Easiest place I can get a relay around where I live in Australia is a place called jaycar, and I think I've found a suitable relay for my video switcher relay: http://www.jaycar.com.au/productView.asp?ID=SY4062 Do you think this will be ok? 2) I'm trying to keep the whole thing as "quiet" as possible, as the unit will be in the dashboard, and preferably, I don't want to be able to hear clicking, etc... I saw a similar post on this forum where someone was asking about quiet automotive relays, and what I brought away from that was that to either use relays rated for lower amps which will be quieter OR use reed relays which are silent. Is that about right? I found a reed relay at jaycar: http://www.jaycar.com.au/productView.asp?ID=SY4032 but I don't think that'll do the job for the SPST relay I need OR the DPDT video switcher relay, as it seems to be only rated to 0.12ma. If I can find a reed relay rated for 12VDC, 3A(both SPST and a DPDT relay), would that give me a quiet solution?