SOLVED - Need help please.. something that seems so simple is driving me nuts - Page 1
Forum section: electronics / beginners

author: Doc38343
author_rank: Contributor
timestamp: June 18, 2015, 06:58:25 pm
content: OK here's my dilema...I have a simple ding dong using 2 - 555 timers works good but I have it wired into my security system panel on the zone for my door.The panel when the door is closed puts out 0v but when it is opened it puts out 6.2V but barely enough current to run the 555 timers, just enough so it works. When the circuit is connected and the door is opened the voltage is only 1.8V to 2.1V. I have been learning and found a simple current limiting on the system mainboard so I understand why the voltage drops and I think that would probably protect the mainboard in the event of a dead short. I do not want to modify anything there and the ding dong works fine when someone opens the door.Currently if they hold the door open my ding dong keeps going and is very annoying. I want to have it do a one or two shot and stop. Sounds simple right. Lots of circuits online for this and even can do a one shot with a 555 timer.... but all of them push the current draw over the threshold and nothing works. I have tried about 50 so far and NONE will work at the very low current. So I discovered MOSFET and got all warm and jiggly thinking it is the answer... nope, I still have the same issue and when I tried the do a simple NPN mosfet to connect the ground of a 9V battery to power the one shot timer and ding dong, it just keeps on rolling and never disconnects. I did it on a bread board triggering a LED to test and some how the 9V battery finds ground whether the mosfet is on or off so I am lost now. WHY???I simply want to use the very low current 2V signal from my door to activate a one shot ding dong that operates under its own power supply, battery or wall nut. I searched and it is like I don't know how to ask the right question because every thing i find is for a push and release button or a circuit that is entirely under its own power. I really thought this was simple thing but DANG! Can someone try and point me in the right direction please. I don't have a PNP mosfet but I was thinking that may be what I need....Disclaimer: I know there are multiple store bought solutions but that is not the point. I want to learn, and overcome this challenge thru design.This is the very thing that got me back into doing electronics as hobby and for my business. I also cannot wire another reed switch to the door it has long since been isolated by construction.ThanksDoc

author: mrkev
author_rank: Regular Contributor
timestamp: June 18, 2015, 07:07:25 pm
content: Even a hand drawn schematic would help us to better understand your situation. It's not really clear like this, at least we could guess what you are doing wrong...

author: Doc38343
author_rank: Contributor
timestamp: June 18, 2015, 11:43:50 pm
content: OK here is my attempt at making an Eagle schematic... first time so I know it could be better done.This works on 9V battery.My VCC is 6.2V but very low current and it is coming from a alarm system board that has current limiting to protect it in the event of dead short. So when the current exceeds the limit it reduces the voltage to protect itself. When I connect this circuit it draws just a bit too much and the voltage drops below the minimum for the gate of the Mosfet and also the 555 timer can't run. The ding dong to the right side without the other 555 and mosfet part actually does work. It is on the border line of drawing too much but just works.I need to be able to connect a lets say 5 or 9V supply voltage then trigger it from the door signal. Sorry I do not know the actual mV it is limited to but would guess in the 300mV range or lower. With no load it is 6.1V when I hook up just the ding dong section it is 2.4 to 2.1 and that is just enough for the 555 to run. (I actually had some 555's that would and some that would not).BTW: since the voltage is so low when the 555's work they sound at a lower frequency than when a 9V battery is used. Sounds more like a beep buzz. (My wife call it a donkey call LOL) When fully powered it is a simple two tone beep similar to a UK police siren.I want it to go off maybe twice and then stop until door is closed and reopened. The problem is that all the simple answers assume that the trigger works like a button press on/off but it is a constant voltage source until the door closes. I started thinking about a relay perhaps so I am now looking for a small voltage one in my salvage boards. I know there is a method to do it using transistors or something I just don't know enough yet to figure it out. What got me was when the mosfet grounds the 9v whether it is on or off, I do not understand, it is 5 to 7 Meg ohm resistance on the big one and I tried all the ones I had available. I may need to do that from the positive side using a PNP mosfet so the ground stays intact and can discharge the one shot capacitor as well as pull the gate to 0V. Almost forgot.... note this if I run the door signal directly to the gate it is 4.1V and does trigger the mosfet.(this was a different setup than the schematic, I was trying to connect and disconnect a 9V battery using a mosfet) Thanks hope this helps.

author: Doc38343
author_rank: Contributor
timestamp: June 19, 2015, 01:24:08 am
content: UPDATE:Found a little 5v relay (JZC-8FA(4181) PCB relay) that can activate down to 2V....Did not work. Here's why the relay won't work at least on the front side. It is an alarm system board so it is looking at a normally closed reed switch on the door. When the door opens it activates the zone. The coil in the relay appears to the alarm board as a closed circuit so it does not activate the zone.Brainfart:: I am going to try and put it behind the one shot 555 circuit (between the two stages and supply only the second stage from the 9V battery off the relay..... has to be a solid state solution.... I got some attiny85's and a arduino board to program it with??? LOL for heavens sake I can't even get a relay or a mosfet to work .

author: adhue587
author_rank: Newbie
timestamp: June 19, 2015, 03:22:12 am
content: Doc,Seems simple enough. You'll want to go with an approach which requires very little current from your alarm panel. An N-Channel MOSFET (2N7000 would work) can be used as a low side switch. You'd drive the high impedance input of the MOSFET with the signal from your alarm panel, would draw very little current due to the high impedance, and have a separate external supply feeding your ding dong circuit. Having a 10K resistor on the gate of the MOSFET to GND will ensure the ding dong circuit stays off when the door is closed (we don't want a floating input).Hope this helps.

author: Doc38343
author_rank: Contributor
timestamp: June 19, 2015, 05:05:05 am
content: Quote from: adhue587 on June 19, 2015, 03:22:12 amHey thanks,OK I have done this exact circuit before EXCEPT for the 10K resistor. So I set it up on a bread board and it works perfectly, LED comes on and goes off as it should. I build it into my actual circuit exactly the same and this is what happens, it sounds funny and pulses at some new frequency like the mosfet is cycling on/off as well as the ding dong for example instead of the long long tone like the normal ding dong it now goes long, half long, short, then repeat over and over. One shot is being restarted by the mosfet cycling. Good news is that when I close the door it stops so that means the mosfet is shutting off correctly now.How do I kill the cycle on the mosfet, I am thinking bigger resistor or perhaps a smoothing cap on the door signal input, it does pulse a little.The part about all this is two things.... without the 10K the mossfet does not switch off and somehow the circuit find ground thru it enough to stay on always. and 2, it does not behave the same when the signal input is from a battery instead of the door signal. If I could connect my OScope to it I would likely see major ripple perhaps even an oscillated frequency as opposed to steady current. I don't have a schematic for the alarm board and wouldn't want it public anyway.My ringing ears and headache is telling me to try a smoothing electrolytic capacitor across the input from the door so I am going to stick a 220uF on it. You did get me closer.ALSO: update from before.... the relay on output is a fail, voltage is just below the trigger level on the output of the 555.

author: Doc38343
author_rank: Contributor
timestamp: June 19, 2015, 05:42:24 am
content: OK the capacitor was a fail....BUT::: It now actually works! I am very happy. I read the datasheet for the mosfet that I actually have and saw there was a difference in the gate to source, so i dug out a 56K resistor and now it works exactly like I want it to. I can use a 9V battery and it will probably last a year... or notI am going to connect it to the power supply wall wart that is supplying the amplified speakers later on. For now I'm just happy I don't have to hear that insane noise all day long. My wife talks to the customers a lot and they stand and hold the door open for some reason.... must be a Tennessee thing..Thanks for your assist it got the job done!Next I will redesign the whole thing into one board and use a 556 for the ding dong and try my first custom circuit board and etching.Eagle Cad is pretty cool for free but I've got a lot of practice to do on it yet.OKEdit:: attached as is working schematic

author: mrkev
author_rank: Regular Contributor
timestamp: June 19, 2015, 06:37:12 pm
content: Well i see that you've solved your problem before I came back from the work.Just a hing on the solution. Always use resistance in gate of the mosfet transistors in this kind of operation.The reason why it didn't work is that the wire acts as a antenna and the mosfet has very high input resistance, so ti most likely picks up noise or 50/60Hz induced voltage from mains (and it gets periodically triggered).

author: Doc38343
author_rank: Contributor
timestamp: June 20, 2015, 03:05:05 am
content: Attached is a cleaned up and revised schematic using 1 555 and 1 556 and circuit board examples from Eagle free.If someone wants the actual eagle files I will be glad to share.I cannot believe how long it took me to learn how to accomplish this simple thing.I learned a ton in the process and can now say with confidence I am the master of this particular circuit and know how to alter the tone speed and frequency as well as the new mosfet one shot section. Thanks guys for posting in these types of forums and helping noobs like myself.I personally really appreciate you all, and I learn a lot from previous posts. I just wanted to follow up so the next guy like me can see it thru to the end.Thanks,Doc