Hi, this is my first post.

I have been working on a mechanism for automatically opening the door to our chicken shed (normal door) for the last couple of weeks and think that it is finally there. It is working on a breadboard, but before I commit it to stripboard, I was wondering if some kind person who has more knowledge than me, could give it a look over to see if there are any potential problems, or missing items. I know for instance that I am not very good at putting in stabilising capacitors etc.

I haven't got very good resources for parts where I live, so am a bit limited. The schmitt chip and ldr are out of a night light I bought at a boot sale (yard sale in US?) so was the 6v electric screwdriver which will open the door. The whole thing has to run off an old moped battery, so had to consume the least amount of power possible.

Thank you in advance

 

Use a cmos 555 instead of the bipolar one. That will reduce the quiescent current.

Your relay switch(es) are not up to the task. The BC548 is only a NPN that is rated for 100mA. Even if it was rated for the relay coil current, the base drive through the 10K is much, much too low. This is an application crying for NFETs.

I would use only a single relay driver, and rig it so that the AND logic is done on the gate. The CMOS circuits up stream can drive the gates of a NFET relay and motor driver.

 

Use a cmos 555 instead of the bipolar one. That will reduce the quiescent current.

Your relay switch(es) are not up to the task. The BC548 is only a NPN that is rated for 100mA. Even if it was rated for the relay coil current, the base drive through the 10K is much, much too low. This is an application crying for NFETs.

I would use only a single relay driver, and rig it so that the AND logic is done on the gate. The CMOS circuits up stream can drive the gates of a NFET relay and motor driver.

Thank you for your input. I will certainly consider using a cmos 555 if I can find one, anything to keep the power consumption low!

It is only a small 12v relay I'm using (takes 42mA when operating), I'm a bit lost after that - I only do this as a hobby, I'm more an electrical engineer than electronic. Could you explain "base drive" and what is a NFET?

Most of what I build has to be sourced from whatever people throw my way or I find at boot sales, am I likely to come across a nfet relay and motor driver in anything scrap?

Thanks

 

Thank you for your input. I will certainly consider using a cmos 555 if I can find one, anything to keep the power consumption low!

It is only a small 12v relay I'm using (takes 42mA when operating), I'm a bit lost after that - I only do this as a hobby, I'm more an electrical engineer than electronic. Could you explain "base drive" and what is a NFET?

Most of what I build has to be sourced from whatever people throw my way or I find at boot sales, am I likely to come across a nfet relay and motor driver in anything scrap?

Thanks

No, but the Arduino/Robot/Maker craze has produced some really cheap reversing motor driver modules that should be low power in standby mode. Since you didn't put in a country when you registered, I dont know what suppliers to refer you to.

 

An NPN used to switch 42mA in its collector should have a base drive of ~ 42mA/10, or 4.2mA. The base drive is being sourced by the 555 which will only pull-up to Vcc-1.5V, so about 10.5V. The voltage across the base resistor is (10.5-0.6)V. To get 4mA, the base resistor would need to be R=E/I = ~10/4 = 2.5K.

I worry about another aspect of your circuit. It might need more hysteresis on the LDR resistance-change trip point. At dawn and dusk, the existing circuit might trip multiple times, especially if there are clouds around...

 

A few points re the circuit :-
1) D1 and D2 can drive the input 6a high, but there is nothing to pull it back down again.
2) The two 47uF caps will drive input 6a above the positive rail voltage and may harm the input protection diodes unless you add a series resistor (say 220k).
3) No 'Open' limit switch?

 

An NPN used to switch 42mA in its collector should have a base drive of ~ 42mA/10, or 4.2mA. The base drive is being sourced by the 555 which will only pull-up to Vcc-1.5V, so about 10.5V. The voltage across the base resistor is (10.5-0.6)V. To get 4mA, the base resistor would need to be R=E/I = ~10/4 = 2.5K.

I worry about another aspect of your circuit. It might need more hysteresis on the LDR resistance-change trip point. At dawn and dusk, the existing circuit might trip multiple times, especially if there are clouds around...

Hi Mike

I have changed those 2 resistors to 2k7, you were right, I hadn't done the math, just being lazy. I tend to just stick in 10k resistors as I've got plenty.

I can appreciate what you are saying about the Arduino unit, but it's not so much that I couldn't get hold of one as afford one, especially just to open a chicken shed door. I live in France btw.

I have had the same concerns about the hysteresis on the ldr circuit, but it is a bit difficult to check it in the real world at the moment, as it is set up on my bench in the basement. I am hoping that the 1/3, 2/3 switching of the schmitt will be sufficient - but I won't be sure until I can test it outside in the sunlight.

I appreciate your time and input

many thanks

Martin

 

A few points re the circuit :-
1) D1 and D2 can drive the input 6a high, but there is nothing to pull it back down again.
2) The two 47uF caps will drive input 6a above the positive rail voltage and may harm the input protection diodes unless you add a series resistor (say 220k).
3) No 'Open' limit switch?

Hi Alec

Thank you for your input.

I don't know about the first one, but the circuit works.

I am totally lost about the second point - how can you get more than the input voltage? Sorry for being a numbnuts

There is no open limit switch because I am just driving the door open for a set time, not to a specific point, as long as it's open enough for a chicken (chook) to pass through, that's good enough. I only put a closed limit switch in, because 1) it's easy to do - I have an old burgler alarm reed switch I can mount on the door, an open switch would be much more difficult to fit as I'm only opening the door about a foot. and 2) The whole point of the 555 timer is so that I don't need to stop the motor with limit switches, if the timer fails on open, then there is no harm done, the mechanism will just disengage itself and keep running until I notice it or the battery dies, if it fails on close then I could overload the circuit or damage the motor, which is why I stuck in the close limit switch.

Hope that is somewhat understandable

Thanks

Martin

 

I am totally lost about the second point - how can you get more than the input voltage?

My bad, I was forgetting the diode's effect.

 

Hi

Just thought I would post an update, in case anybody is interested (probably not lol).

After much swearing, circuit altering and more swearing, I have finally got it working satisfactorily. The first problem that I had was my idea to drop the voltage down from 12 to 6v for the drive using resistors didn't pan out too well, I then tried a voltage dropping circuit "borrowed" from a mobile phone car charger, but it wasn't up to the task, so I finally settled on using diodes (I am a great believer in the idiom K.I.S.S.). I got it working on the bench, but had the occasional hiccup with it opening twice, but then it seemed to cure itself. On mounting it in place it all worked ok, except it began opening twice, sometimes three times in a row. I tested everything I could think of, but without an oscilloscope, couldn't work out what was triggering the additional openings. My last idea was emf from the motor, so I stuck a large capacitor (not polarised) across the motor terminals, this appeared to fix the problem.

Another problem I didn't foresee was that for some reason, the door was opening slightly faster than closing (maybe it was easier to open, or the drive works better one way?), which because I had them running on a timer, resulted in the the door being left more open each evening! I kind of bodged that problem by fitting an extra diode in the closing circuit.

All worked well for 3 days and then I had the extra opening again (aaargh!) so running out of patience managed to fit an open limit switch I had hanging around (a bit chunky and over engineered, but does the job) - problem solved. Still not sure what was causing the problem, maybe the coil on the relay? It is only operated when the door is opening, but I did have a flywheel diode fitted across it.

Anyway, have uploaded some pics and the final circuit, feel free to comment (nicely please), and thank you to those who helped me above.