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  • Author Author: jw0752
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Reverse Engineering an Alarm Siren Module

jw0752

One of the things that I enjoy doing is taking a small one or two sided circuit board and creating a schematic of it. Today I have a circuit board from a 105 db piezo siren for which I am going to create a schematic.  Here is the Module.

 

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The first step is to get some paper and choose a starting point. In this case I picked the output transformer and then worked my way back. As you can imagine the board must be flipped back and forth and ones perspective must constantly shift from topside to mirror image and back. The resistors must be read and measured if they can not be seen. This is also done with the capacitors. On this board the identification of the small green mylars were face to face and so one had to be removed to determine the value. When the rough draft of the schematic is finished it is never very pretty. Here is my rough draft.

 

 

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This is obviously not very good or very clear. In order to not put too many line on this draft the ground line was replaced with ground symbols and one junction point was labeled #3 and a second junction point was labeled *. What happened to #1 and #2? I am not sure it just seemed ok at the time to call them * and #3. After the rough draft is finished I use it to produce a more conventional looking hand done draft. Here is what it looks like after it has been cleaned up.

 

 

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This one while still not very neat at least looks more conventional and at this point I also add component numbering. I also take close up photos of the board and label the components to match the schematic. This serves to tie things together and to make certain that I have not left anything out of my schematic. Here are the pictures and labels that correspond to the schematic.

 

 

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Now I take all the drafts and produce a CAD drawing of the schematic to complete my file on this particular circuit.

 

 

 

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This process is good exercise for the brain and gives me a very good understanding of the circuit by the time I have finished.

 

John

Parents

  • Hi

    I am not an expert in reverse engineering so forgive me if this is a stupid question. How do you measure resistors and capacitors while they are mounted on the pcb? Aren't all connected components going to interfere with the measure?

  • in reply to amgalbu

    I forgot to address capacitors. Capacitors are usually not readable in the circuit. You can test them to see if they are shorted but the interference from peripheral components will mess up an actual reading of capacitance. Here the only option is to read the label or remove the part and then test it.

    John

  • in reply to jw0752

    Here the only option is to read the label or remove the part and then test it.

    Yep that is sometimes the only way.

     

    Something to note here (since you've raised a really good subject) is often the purpose is to obtain a schematic in order to fix whatever it is.

    Sometimes the value isn't required.

     

    While we are used to doing this on thru hole components, I'm afraid our days might be numbered with SMD parts, as many of the smaller ones have no markings at all. image

     

    maybe you might need another blog on the business of fixing something that there is no circuit for.

    If you're like me ... you start by noting what it does/doesn't do, then looking for obvious damage, then working from the end back to the start.

     

    Mark

  • in reply to jw0752

    Thank you very much!

    Very clear explanation!

  • in reply to mcb1

    Yes Mark you are correct on a number of counts. With the advent of multi-layer boards and SMD parts I can see the end of the trail for people like myself who like to repair circuit boards. Through the hole, one or two side boards were understandable and repairable. I come from an industry, dental bio-medical, where there general rule was not to repair circuit boards but replace them at upwards of $1,000.00 a piece. My shop was a rebel and we repaired them. In situations where the boards were part of a popular piece of equipment we went years in many cases without having to replace a board. Most of the time, due to design weaknesses, the boards would fail with the same problem or problems. The learning curve was to get to know the board well and subsequent fixes were then almost intuitive. I get a lot of satisfaction out of beating the system on things that are designed unserviceable. I like to take switches apart to clean their contacts and reassemble them and I will try to fix things that probably should just be thrown away. One short story before I go. When I was new to the dental service business I came across a dental x-ray unit that was not working. I became convinced that the problem was in the x-ray head. This is in most cases considered an unserviceable part but following my mantra I took out the screws and cracked the case. As the case separated there was a hiss of air from the head. I called an experienced acquaintance at this point to ask what I should do next. He listened to my story and laughed heartily at me. He Said, "Don't worry, just make your repairs to the head and then before you put it back together gather up all those Xenon atoms that escaped and put them back."

    John

  • in reply to jw0752

    John

    I will try to fix things that probably should just be thrown away

    Unfortunately we have produced a throw away society, and I'm not convinced we are any better for it.

    Price is the aim, rather than reliability and good design, which doesn't help make better Engineers IMO.

     

    Nice story .... imageimage

     

    Mark

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