Sockets are your friend, unless it is in a high vibration area.

Well right now the circuit is on a solderless breadboard. Once I know that it is working correctly I will make a PCB for it and I will use a socket so that I don't have to worry about overheating the pins when I solder it.

 

Something else that occurs, MOSFETs do not have standardized pinouts. Do you have a datasheet?

 

Something else that occurs, MOSFETs do not have standardized pinouts. Do you have a datasheet?

The MOSFET was a radioshack special. The packaging showed the pinout. I am pretty sure it was what I have usually seen which I think is when viewed from the front going from left to right Gate, Drain, Source.

 

Is a 1N4148 sufficient for my flyback diode? It just happens to be what I have on hand.

 

Nope, that is a high speed low power diode, meant for digital functions. I use them to make logic. You need a power diode, something like a 1N4001 or better. The motor takes in a lot of power, it will kick out a lot of power.

 

Nope, that is a high speed low power diode, meant for digital functions. I use them to make logic. You need a power diode, something like a 1N4001 or better. The motor takes in a lot of power, it will kick out a lot of power.

I hear ya. I bet I can pilfer one from one of the numerous circuit boards that I have ripped out of various electronic products over the years.

 

I don't see any bypass capacitors for the 555 timer supply. They are not optional. If you read National Semiconductors' datasheet for the LM555 timer, you will find that the minimum requirement is a 0.1uF and a 1uF or larger across the power (Vcc or Vdd) and ground pins.

As has already been mentioned, I prefer to use smaller timing caps with larger resistors; I'd replace the 10k and 220uF with 100k and 22uF or 1MEG and 2.2uF.

Pin 7 of the 555 timer does have a 15mA current limit, although this is frequently abused by most hobbyists. If you really wanted to follow the datasheet specifications to the letter, you would add a current limiting resistor between the timing capacitor and pin 7; Rlimit >= Vcc * 2/3 / 15mA = 622 Ohms or greater. This will affect the timing slightly, but not very much.

 

I don't see any bypass capacitors for the 555 timer supply. They are not optional. If you read National Semiconductors' datasheet for the LM555 timer, you will find that the minimum requirement is a 0.1uF and a 1uF or larger across the power (Vcc or Vdd) and ground pins.

As has already been mentioned, I prefer to use smaller timing caps with larger resistors; I'd replace the 10k and 220uF with 100k and 22uF or 1MEG and 2.2uF.

Pin 7 of the 555 timer does have a 15mA current limit, although this is frequently abused by most hobbyists. If you really wanted to follow the datasheet specifications to the letter, you would add a current limiting resistor between the timing capacitor and pin 7; Rlimit >= Vcc * 2/3 / 15mA = 622 Ohms or greater. This will affect the timing slightly, but not very much.

I will implement these changes to the design. Unfortunately I haven't had a chance to do anything with it over the last few days. The kids are keeping me too busy. Hopefully make the changes and try it out again this weekend.

 

It's actually kind of interesting about the bypass capacitors across Vcc and Ground because the TI datasheet for the NE555 doesn't mention it anywhere and doesn't include them on any of their sample schematics. The National Semiconductor datasheet for the LM555 (which is labeled as a direct replacement for the NE555) does include a small note at the end regarding the caps but also doesn't show them in any of their sample schematics. Thanks for pointing this out to me SgtWookie as I am guessing they are important.

 

They can be. When they are needed they are a showstopper. On small single chip designs with a 9V I don't bother. Yours are connected to a power supply that could allow noise through though. It is a situation of can't hurt, might be critical.

About large timing caps, this is something new, and I place it in the category of myth. The only difference between a 10,000µF and a 10µF is duration, not current, and I have never had a cap blow a 555 through pin 7, ever, nor have I ever seen it hinted at for the data sheet, and I have built many 555 circuits, some with 10,000µF (4700µF tends to be my usual max). The problem you may run into with large capacitance (and it is rare) is leakage. I'm not sure where this came from frankly.

 

They can be. When they are needed they are a showstopper. On small single chip designs with a 9V I don't bother. Yours are connected to a power supply that could allow noise through though. It is a situation of can't hurt, might be critical.

I don't think I mentioned it but my power supply is just a 12V 4.5 Amp Hour SLA battery so I think the power supply is clean. Still I know that when the output changes states you can get a jitter in the power supply so I would assume the cap would clean that up.

 

Yep, and to a large extent so would the battery. Batteries and capacitors look very similar electrically. Like I said, you can't go wrong adding capacitors, but sometimes it is overkill.

 

They can be. When they are needed they are a showstopper. On small single chip designs with a 9V I don't bother. Yours are connected to a power supply that could allow noise through though. It is a situation of can't hurt, might be critical.

I'm simply quoting National Semiconductor's datasheet - but it's a known that when a 555's output toggles state, that there is a momentary direct short across the supply. If the power supply leads are of any significant length, the momentary short could cause the inductance of the supply leads to oscillate. Having bypass caps of the recommended sizes in place eliminates that possibility. It is better to have them in place and not need them, than not have them and need them and wonder why it doesn't work.

While you and I wouldn't use long, skinny power leads to a breadboard (or would we? ) many of the newbies would not know that might affect their circuit operation.

About large timing caps, this is something new, and I place it in the category of myth. The only difference between a 10,000µF and a 10µF is duration, not current, and I have never had a cap blow a 555 through pin 7, ever, nor have I ever seen it hinted at for the data sheet, and I have built many 555 circuits, some with 10,000µF (4700µF tends to be my usual max). The problem you may run into with large capacitance (and it is rare) is leakage. I'm not sure where this came from frankly.

I can't remember exactly how I figured it out before offhand. However, if you look at National Semiconductor's datasheet for the LM555, July 2006 version, on page 6 bottom right plot, you'll see the saturation voltage vs current over temperature for pin 7. The plot stops at about 18mA & 180mV for -55°, and somewhat higher (~400mV to 500mV, 30mA) for 25° and 125°C. Usually, when data is not plotted for a range, it's out of the range of the device. As 18mA was the lowest common denominator, 15mA should certainly be safe.

Above that on page 4, there is a note which reads:

Note 7: No protection against excessive pin 7 current is necessary providing the package dissipation rating will not be exceeded.

Now, the package power dissipation is subjective - depending on which package is used and the environment it's used in.

Power Dissipation (Note 3)
LM555CM, LM555CN 1180 mW
LM555CMM 613 mW

And:

Note 3: For operating at elevated temperatures the device must be derated above 25˚C
esistance of 106˚C/W (DIP), 170˚C/W (S0-8), and 204˚C/W (MSOP) junction to ambient.

JAN versions have lower limits for power dissipation depending on packaging (~325mW range).

 

Thing is, these durations are short, very short. I have seen applications where people use it for continuous DC, which is not a good idea. Capacitors in general will discharge fairly fast and with similar current profiles. That same datasheet showed the standard circuits a 555 uses I'll bet, the basic designs are pretty set and well tested.