Received a request by a new member "Jure George" requesting help with a modification to a circuit I posted. I have been talking to him via email, but wanted to make the circuit idea public here so that I don't go and cause him to buy parts he doesn't need, or to design a circuit that won't work. Here is the info for all to contemplate and aid in designing...

Hi, I would like to know if you would be able to modify the Circuit you have posted for the 10 sec. Timer using a 555 and a push button trigger with a relay in the Circuit.

I would like a Circuit with the same Variable operating time of 0 Sec. to approx. 15 Sec. Triggered by a small level signal, 5v. No relay needed But circuit must be able to drive small motor 1-2 A. Main battery Supply 6v - 12v max.

Could you please send me the circuit plan ?

Thanks,

Sorry to reply so late...

Let me get this straight:

You want the 555 timer to be triggered by a 5V logic signal. Will it be momentary, +5V, or -5V or do I have a choice?

Below is start, but may need some rewiring for the trigger. As it stands now, a momentary push brings Pin 2 of the 555 to 0V and thus starts the timer. The Motor you are driving, is it AC or DC?

Hi iONic
First Thanks for replying to my question. The circuit I need would be triggered by a Positive 5v and the Motor would be D.C. OR whatever else could be used. So If I have listed all my needs below please let me know:


- 555 I.C
- Main D.C Battery Supply 6v - 12v max. ( Common Battery values used 6v, 7.2v,7.4v,12v )
- To Drive a D.C motor, max. 1-2A
- Variable Opperational time ( Time it Runs ) 1 Sec. Motor On - 15 Sec. Running
- Positive 5v Signal used to trigger the start of the 555 Timer Circuit
- Fused to protect Circuit from D.C Motor failure


I think I have given you all my specs I need. Please let me know if you can modify the Circuit to work with these Specs. All this for a Circuit to be used in Remote Control Boats / Ships and Submarines ? The Smaller the physical size of the circuit, the more likely it will fit in a r.c submarine and small r.c boat.

Look forward to seeing your circuit.

Thanks,

See Circuit diagram below.

 

There is no current limiting resistor on the base of the 3055 transistor. You'll fry the ICM7555 timer. Even at maximum voltage, it can only source about 10mA.

The ICM7555 timer cannot source enough current to saturate a 3055 transistor. Replace it with a logic-level MOSFET, with a suitable resistor to limit gate charging current to 10mA max, and use a 10k pull-down resistor on the gate to keep it turned off in case of circuit failure. Add a flywheel diode across the motor.

You have a 2A supply and a 5A fuse. Instead, you should be using a 2A slow-blow fuse.

A 150uF electrolytic capacitor will probably have too much leakage current to be charged up via the 100k pot.

If the 100k pot is set too low, it will get burned up due to the current flowing through it trying to charge the cap.

A 7404 inverter is not necessary. Put a 10k pullup resistor on the ICM7555 pin 2, and use a 2N2222 NPN transistor with it's collector connected to pin 2, emitter to ground, and a 4.7k resistor to the signal input. When the signal input exceeds about 0.7v, the transistor will conduct, placing near 0v on pin 2.

 

A weak but passing 2N3055 transistor needs a base current of 400mA (40 times more than the output current from an ICL7555) for a collector current of 4A.

An ordinary 555 has a max ouput current of 100mA (10 times more than an ICL7555)when it has a supply as low as 5V and still won't drive many 2N3055 transistors.

 

There is no current limiting resistor on the base of the 3055 transistor. You'll fry the ICM7555 timer. Even at maximum voltage, it can only source about 10mA.

The ICM7555 timer cannot source enough current to saturate a 3055 transistor. Replace it with a logic-level MOSFET, with a suitable resistor to limit gate charging current to 10mA max, and use a 10k pull-down resistor on the gate to keep it turned off in case of circuit failure. Add a flywheel diode across the motor.

You have a 2A supply and a 5A fuse. Instead, you should be using a 2A slow-blow fuse.

A 150uF electrolytic capacitor will probably have too much leakage current to be charged up via the 100k pot.

If the 100k pot is set too low, it will get burned up due to the current flowing through it trying to charge the cap.

A 7404 inverter is not necessary. Put a 10k pullup resistor on the ICM7555 pin 2, and use a 2N2222 NPN transistor with it's collector connected to pin 2, emitter to ground, and a 4.7k resistor to the signal input. When the signal input exceeds about 0.7v, the transistor will conduct, placing near 0v on pin 2.

Attached is the best I can do tonight....

 

Go back to a CMOS timer. A bjt 555 will have too much of a voltage drop on it's Darlington output to get a logic-level MOSFET gate high enough (Vcc-1.3 to Vcc-1.7).

Replace C1 with a 2.7uF poly cap (for low leakage).

Replace R1 with a 5 MEG pot, and add a 100k fixed resistor either above or below it; in series with it. This will give a range of about 300mS to 15 seconds. Mouser carries 5M pots:
http://www.mouser.com/Search/ProductDetail.aspx?qs=60Hxy/TM9vQyFpXQXEQufw==
So does Digikey, but they're much more expensive for this particular item.

Use a 470 Ohm resistor between the gate and the 555 pin 3 (timer output). In conjunction with the 10k pull-down resistor, about 95% of Vcc will be achieved on the MOSFET's gate. Since this is a manually-triggered, very low PRF circuit, the time the MOSFET spends in the linear region isn't that important, but even with the high resistance and 10mA limit to the gate current should be under 10uS.

For the motor flywheel diode, a 1N5400 series could be used, cathode towards the supply voltage. A Schottky diode would be better, but axial-leaded Schottky's are getting a bit hard to find. However, Mouser has these in stock:
http://mouser.com/Search/ProductDetail.aspx?qs=KFo7JewZbUFlhstWnyA22Q==
2A 40V Schottky barrier diodes in a DO-41 axial package, easy to deal with. Pretty reasonable too, at $0.18/ea when you're just buying a few of 'em. Get some for spares.
Datasheet is here: http://61.222.192.61/mccsemi/up_pdf/SR202-SR206(DO-41).PDF

They'd be good for building a bridge rectifier if you wanted to build a power supply from discrete components.

 

For the motor flywheel diode, a 1N5400 series could be used, cathode towards the supply voltage. A Schottky diode would be better, but axial-leaded Schottky's are getting a bit hard to find.

Why would it be better to use a Schottky diode instead of a normal power diode on a motor?

 

Replace C1 with a 2.7uF poly cap (for low leakage).

Replace R1 with a 5 MEG pot, and add a 100k fixed resistor either above or below it; in series with it. This will give a range of about 300mS to 15 minutes.

Sgt. Am I still sleeping or is this a tyop? 300ms to 15minutes? Should it not be 300ms to 15seconds?


Updated circuit bellow...

 

Why would it be better to use a Schottky diode instead of a normal power diode on a motor?

Standard rectifiers, like the 1N4000 or 1N5400 series, switch very slowly. This gives the reverse EMF voltage lots of time to build up to possibly high levels, which would blast the source terminal of the MOSFET. I'm assuming the motor is a brushed type, where commutator connections are made/broken thousands of times per second.

If the current were very low, our OP could've used switching diodes, like 1N914 or 1N4148. These are hundreds of times faster than standard rectifiers.

However, the 2A current would burn the switching diodes up in a heartbeat. Schottky diodes have both very low switching times, and much lower Vf than standard diodes. The latter is not very significant here, but the switching time is.

Our OP could use a standard switching diode if they used a small ceramic capacitor across it; perhaps 220pF-1nF. This would slow the rise time of the reverse EMF pulse, buying time for the diode to conduct.

 

Am I still sleeping or is this a tyop? 300ms to 15minutes? Should it not be 300ms to 15seconds?

You typo'ed "typo"

Yes, it's 15 seconds, not 15 minutes. I was distracted when I made the post.

You need a 100k fixed resistor in series with R1. Otherwise, if our OP turns R1 to minimum resistance, they'll fry R1, the 555 timer, or both.

If you're going to leave the 1N5400 series diode across M1, then add a 330pF ceramic cap across the diode.

The Schottky diode I recommended doesn't need the 330pF cap.

 

Standard rectifiers, like the 1N4000 or 1N5400 series, switch very slowly. This gives the reverse EMF voltage lots of time to build up to possibly high levels, which would blast the source terminal of the MOSFET.

While a Schottky diode is fast in recovery, the diode in this case is not recovering. It is reversed biased all the time and start conducting when the motor inductive back emf becomes larger than its forward voltage.

Whether it is switching slower in the forward direction is not known for sure but I would refer you to the following website in which the author took a test to measure how fast IN4xxx & Schottky diodes switches in both the forward and recovery conditions.

Makes interesting reading.

Diode Turn-on Time measurement

 

While a Schottky diode is fast in recovery, the diode in this case is not recovering. It is reversed biased all the time and start conducting when the motor inductive back emf becomes larger than its forward voltage.

Whether it is switching slower in the forward direction is not known for sure but I would refer you to the following website in which the author took a test to measure how fast IN4xxx & Schottky diodes switches in both the forward and recovery conditions.

Makes interesting reading.

Diode Turn-on Time measurement

Yes, it is interesting reading. They switch on more quickly than I thought.

But, note the "spike" when the 1N4005 and 1N4007 become forward biased. There was practically no visible spike in the 11DQ05 Schottky test.

It's too bad he didn't perform the test actually using the test diodes as a clamp for a coil.

Come to think about it though, in this particular application the MOSFET is going to take around 200nS-500nS to turn off, so the rise time should be fairly slow from that aspect - it's the commutator switching that will happen quickly.

I feel that it would still be a good idea to use a small cap across the diode; as it would completely absorb that spike.

 

Think I'll wait till the OP shows up and gives us his opinions.

 

Hello I am the person who asked for help to modify this Circuit

My needs are for a Timer Circuit to be triggered by a Positive 5v signal, run for a dialed in amount of time, from 1 to 15 Sec. max. It would run a small brushed D.C motor with max. of 2A but usually below 1A. The entire Circuit would run off of the main boats Battery, either 6v, 7.2v, 7.4v upto 12v. Also the circuit needs to be physically small in size to fit smaller boats OR submarines.

Also I would like to use the 555 I.C and NO FETS please because Airflow cooling most times is not possible. I have used a 2N3904 or 2SC1815 in another circuit with a 555 I.C before but looking for more durability in a design.

I wish I new how to post Circuit Drawings I have drawn on paper here for your thoughts and suggestions. I am OLD SCHOOL, paper pencil/pen ruler design and then bread board experimenting the circuit to see if works.

Any help would be appreciated as I have some more circuit I am having trouble with also.

Thanks Again to All,

George

Hope that helps you with what I need.

 

If you want the minimum to be 1 second, then the 10k resistor in series with R1 needs to be increased to 330k.

The MOSFET won't require cooling. A BJT (transistor) solution would require a Darlington configuration, which would require cooling.