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555 timing chip help


R_U_AFA

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I am trying to use a 555 chip in monostable mode to control a pulse to operate a motor.

I will be powering it from the standered car battery, so to get the current down to the required 2000uA, I will have to use a resistor around 7000ohms

What I want know is does this resistor have to be added in to the resistor/capacitor caculation for the timeing interval.

Or Can the resistor at  R1 now be ommited

 

 

First picture is the original diagram I used as a base for the circuit

Then second picture with my modifications (caculations for R1 and C1 yet to be worked out)

 

SAM_0182.JPG

SAM_0181.JPG

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Current limiting resistor will not affect the timing calculation, R1 will still be needed.

 

FWIW I dont think you need a current limiting resistor if the timer IC is a 12v logic chip but its been 30 years since I was constructing circuits and learning along the way so others will advise.

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Thanks

With regards the current, I'm just going on the specifications on the ebay page where I purchased it.

I must admit it does seem rather low to me, but I don't want to burn the chip out (Being a total novice I just want be careful)

 

 

555 chip instructios - Copy.jpg

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Surely the 2000uA is what the 555 will draw internally from the 12v battery when it is directly connected, not including the output current, and it will not need current limiting. I have used these ICs in various circuits in the past on various voltages and I gave never used a current limiting resistor. The value that you really need to be concerned about is the maximum output current so that you do not overload the chip or overheat it if it us in an enclosure.

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So A few problems today

I created circuit as shown above (except now with a 100ohm current limiting resistor)

Using 5600ohm resistor at R1 and 335uF capacitance at C1(made from 2x 470uF in series and 100uF in parallel with them)

As far a the time caculation as I understand It is:  Time = 1.1xR1xC1

SO Time= 1.1 x 5600x 0.000335 witch equals 2.0636 seconds (close enough to the 2.05 I want)

 

But what did happen was

1. circuit stars stable no output at pin 3  (OK)

2. drop current at pin 2 output at 3 goes high (OK)

And this is where the problem is, one of the following is happening, I'm not sure which

3a. output at 3 stays high and doen't return to low even after 5 mins  (should return to low after 2.05 s, and stay low)

OR

3b. output at pin 3 goes high low repeatedly as to seem like a constant flow, and still does'nt return to a stable state (? shouldn't happen pins 6 & 7 connected together)

 

Is there something here I have wrong?

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Which of the 2 circuits that you put in your first post have you built?

Can you please explain exactly what you want the circuit to output in therms of of high and low states and timings from startup?

What are you using to check the output?

Are you keepimg the output current below 200mA?

Does the chip get hot when it is running?

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Which of the 2 circuits that you put in your first post have you built?

The 2nd one that has been editied

 

Can you please explain exactly what you want the circuit to output in therms of of high and low states and timings from startup?

High state: Enough to switch on a transistor, probably anywhere above 2v and 100mA

Low state: As close to 0v as possiable

Timed pulse of 2.05 seconds after pin 2 goes low

 

What are you using to check the output?

Nothing, just the internal resistance of the tranistors base-emitter path

 

Are you keepimg the output current below 200mA?

Sorry, don't know, didn't measure that

 

Does the chip get hot when it is running?

No

 

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Describe exactly what you are aiming to do.

 

Hand draw your complete circuit with the output transistor etc, I can suggest a correct circuit design based on your 555, with appropriate additional components to protect it in an automotive application.  In particular show what load you are driving and how (bipolar/mosfet etc) and why 2.05 seconds is critical.

Also describe the input trigger signal from your external control unit, high/low voltage levels, duration of trigger pulse supplied, high to low edge trigger etc

 

The 100 ohm resistor you connected in line with the battery drops voltage if any current is taken from pin 3, and so creates an unstable supply voltage to the IC and its timing side RC so it will have unpredictable behaviour, such as oscilliating. Supply voltages to ICs should always be stable and (capacitor) decoupled to ensure correct operation.

 

NE555 is a 40 year old design, there are more recent CMOS equivalents that have much lower power draw, ICM7555 etc 

 

You have selected a rather ridiculous RC arrangement, electrolytic capacitors are not great in timing applications, most have ±20% tolerance and leakage is a problem in high capacitance electrolytics. 

 

Before any one asks for my qualifications, I have active been in industrial electronics/micro design for almost 50 years.....

 

 

 

Edited by xman
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It us a long time since I used a 555 but here ate my thoughts.

 

If you are using your second modified circuit then you need to connect the trigger pin (2) to ground temporarily to start the timing and connect that pin via a resistor, as per the first circuit,  to the +ve rail otherwise it will float between high and low and give strange results. Do that and see whether the output is consistent. You could build the first circuit and make sure that it works with a push button before modifying it.

 

If you are using polarised capacitors make sure that they are the correct way round. I do not know how your capacitor setup will react as I have never done them in series and parallel at the same time. I would suggest that you just use a 470uf capacitor to start with in order to make trouble shooting easier.

Edited by Liger1956
More info.
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Yes, the trigger must not float. It must also return to high state before the output returns low.

Also the trigger threshold voltages must be exceeded (<27% VDD to trigger, return >72% VDD) this may be a problem if the input trigger voltage does not comply (drawing suggests a 9 volt source) in which case an additional voltage translation stage may be required.

Edited by xman
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To start off with I will just say thanks for your help.

I will upload sketch circuit in a few mins

To expand more I will start at the very begining

So my car has c/l but only on the key, so decided to upgrade to remote c/l and got one of the readly available aftermarket kits online.

I have had no problems intergrating this with the cars current c/l when on a test fitting.

It was after this that I noticed the optinal remote boot release optinon like on most new cars, and I thought it would be as simple as a few more wires and a selonoid (late model felicia's already have a manual boot release mechenisim in place to work with)

Now this is where the problems start; the springs in the current manual release mechenisim are too strong for selonoid to actuate, and with limited space avaliable my only option was a motor with a small arm to gain some leverage.

So still should be quite simple, a relay driven by the pulse emmited from the remote c/l unit. No not that simple the pulse form the unit is pitifully small somewhere around 25mV and 2mA for 1.8 seconds

Again this can be worked around using an op-amp as an amplyfier to trigger a transistor witch triggers a relay (it took up a lot of time working this circuit out) I will include this in my sketch

Now this is the problem solved you would think, but again No operating the motor for 1.8seconds can leave the boot release mechenism in the unlocked posison and the boot being unlatched. Now this could be solved by pressing the key fob's boot relese button untill the boot mechensim is latched again, but roughly speaking the mechensim would land in unlatched posison 25% of the time.

So I figured by using a 555 chip I could have the motor always end in a place that leaves the boot latched (2.05seconds leaves the motor in the same place every time, a park posision if you will)

Now with the back story over lets continue....

 

There is quite a lot to take away from your posts, there seems to be quite a few mistakes I have made

Lets start simple, with the term float. Yes, the trigger must not float can you elaberate a bit more?

Also you say Supply voltages to ICs should always be stable and (capacitor) decoupled to ensure correct operation. So am I missing some capacitors, and does the 100ohm resistor need to go?

I understand that the capacitor setup I have is silly, but I was trying to make up the correct capacitance for the timing with the bits I have. Would you suggest a higher value resistor and lower value capacitor?

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Here is the circuit exactly as I had it today(but withot the motor connected)

Both transistors are bipolar NPN transistors BC547B

The input trigger High is 9V 28mA constant, The input trigger low I will measure tomorrow and update(I seem to remember it is somewhere near Zero for 1.8 seconds)

There is also a fuse on the motor side of the relay have forgot to draw

 

SAM_0183 - Copy.JPG

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Only just seen your edit xman

The trigger return voltage at pin 2 is boarderline (72% of 12V 8.64V) but if (72% of 12.5V = 9V)

Would it be posiable to use a voltage divider to bring down the supply voltage?

I will have to do some more measurments tommorow

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Well there's quite a lot wrong with that design. It would help if you could supply further information.

 

1. Some info about the C/L unit - a spec sheet, link to a webpage, part number whatever. I've a feeling you've missed that the output you are using to trigger is an open collector type output.

 

2. Relay info - coil resistance and operating voltage (or a part no)

 

3. Not sure what that diode is doing in the op-amp feedback, is it a diode or a LED?

 

4. Opamp part no.

 

I must pass comment that I think its highly optimistic for you to think your motor will always park in the same position, given variables like voltage, load, temperature etc and what would you do if it stops in the wrong position but once I've got the info above I'll sketch out a circuit that should work as you originally wanted based on the bits you already have.

 

I'll add some notes to explain various things you may not be aware of that you need that are missing.

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So I have been messing about again today and took some of your advice on board, and GOOD NEWS I have got the circuit working pretty close to how I want it (but only tryed it out a few times)

You will see from the sketch below, I have modified the circuit again with a extra relay, and now the 555 side looks a lot like the 1st diagram I uploaded to start with.

I am still using the rather convoluted RC arrangement from earler although not shown it in the sketch below(mabey something to work on there) and the timing is a bit out around 0.1 seconds too long (I am guessing that could be due to issues mentioned in your earler post about tolerances)

Xman I can still answer the questions above if you want to point out a better way to do it/where I have gone Wrong

 

 

 

Untitled 2tryrelese - Copy.jpg

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Sorry got the relays the wrong way around

Relay 1's 30 to 87a pathway is open when 85 86 energised

relay 2 is oposite motor circuit is closed when energised(allowing the motor to run)

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First let me advise you - do NOT take any current measurements - you risk blowing components, it may be the reason the remote c/l unit is only outputting 25mV as you may have blown a transistor internally. We only need voltage measurements.

 

I am trying to establish a few fundamental things first, assuming the remote unit output is not damaged, it may have what is known as an open collector output, to test for that, please follow the instructions in the diagram below and post your results. If it is as suspected then a simple pull up or pull down resistor will give us a large healthy signal.

 

Image.thumb.jpg.9d4b874037eda4e1e688581bf408ede4.jpg

 

Your design is over complex with many errors, the transistors (underrated btw) wont last very long switching inductive relay coils without protection diodes, to name but one error. But if we can establish what the remote unit output is doing then I'm sure we can sort out a much simpler circuit.

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So compleated the test in the previous post, and results are not as expected:

1. Volts with remote idle = 6V

but

2. Volts with remote active = 6V

(I am wondering if something has been damaged internally, because I have measured current previously)

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  • 4 weeks later...

So this project has been on the backbench for a while, But I got back to it today:

The circuit has been modified again with a transistor NOT gate added in place of relay 1, and I have it working pretty much as it should, as many times as I want, when no load is placed on the relay(relay switches on off as it shoud) as shown in first diagram below.

 

But now a new problem seems to have raised its head, that when a load is connected to the relay(as in the 2nd circuit diagram) and the pulse is sent, the c/l control unit goes quiet/stops working when the relay switches. The circuit does work, but just once, because the control unit is now off, and no further pulses can be sent. It is almost as if the control unit power is removed/bypassed or goes too low, when relay switches.

Now I am sure that there is an obvious fundamental principle of electronics that I am missing here, or a reason that this is happening.

 

N. B. I also tryed a different way as shown in the 3rd diagram using a seperate battery, and it works perfectly every time, but of cause I can't add an extra battery to the car to solve this problem.

 

fenomotob2d 2tryrelese - Co - Copy.jpg

feb2d 2tryrelese - Copy - Copy - Copy.jpg

dobbat feb2d 2tryrelese - C - Copy.jpg

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  • 3 months later...

At a quick glance I'm struggling to work out what you are trying to do. First a 555 will work up to 15v DC and will supply up to 200mA to drive an external circuit, so it will work from a car (12v) battery without problems. However, if the other equipment needs a lower voltage, might I suggest one of the three terminal regulators (7405/7409) to drop the circuit voltage to that required. The 200mA output will drive most small relays and again if you need more power- add another relay (or stick in a power transistor to feed from battery). 

 

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