555 on/off circuit for 5v motor (w/ schematic)

[CDRIVE]

That's great, thanks. Outputs 5.5v which is perfect for 3 - 6v motor. At first with your revision, I was frying yet another NPN until I decided to remove the connection between between collector and pin 8 (V+), thinking that putting 2 V+ connections to 1 collector wasn't a good idea. Then everything worked great.

I have no idea what you said but I think it translates to connecting the collector directly the Vcc and emitter to GND. If so there's no mystery why the transistor popped.

Glad it's working now but it's time to trim the base resistor to a proper value for good saturation. The base resistor value is commonly calculated to provide 1/10th the collector current. So the first step is to measure the steady state current of your motor when connected directly to the battery.

Chris

 

[HellasTechn]

One thing i would like to point out here is that the 555 circuit is not reccomended for frequencys smaller than 1hz.

a cheap microcontroller like the picaxe 08m2 would be a good choice for that project. The code for such task is extreamely easy and the outcome would be very satisfying.

 

[p1ne]

Chris thanks for that tip; collector current is around 120 mA and base resistor current around 10 mA with 470 ohm resistor, so that seems like good saturation. I won't connect motor directly to 7v power because that will fry motor. There must be enough voltage drop with diodes and NPN that I'm getting around 5.5v on output.

Hellas, I've got an Arduino and I've read about similar application using Arduino triggering NPN. First, I want to learn something about PWM using discrete components.

Would someone please explain: 1) why no discharge 7 pin connection 2) how the 2 diodes connected to 20 (on), 100K (off), and trigger (2) and output (3) are all working together and 3) what the electrolytic capacitor is doing. I see that the capacitor is getting charged while circuit is "on" ie. current makes it past the 20K diode to capacitor, and then gets blocked before the 100K "off" resistor, so current has nowhere to go but to charge the capacitor. So that means the capacitor must discharge while circuit is "off", and it's discharging into the 100K resistor. But what's the effect of that?

I can also see that the 2 pin trigger connection from 20K diode is like an "input" that sets in motion the output to pin 3 that will activate the NPN. But how does the "switch" to "off" then come about? I see the output pin 3 also going to 20/100K junction, but how is on/off determined? I notice that when connecting power, the motor goes on right away. Thanks.

 

[CDRIVE]

You can also improve the circuit by placing the transistor between the motor - terminal and ground. I.e.

motor + to V+
motor - to collector
emitter to ground

This will reduce the voltage drop across the transistor to about 0.2V and make the transistor run cooler.

Bob

Do you remember this post from Bob? His last sentence is what we strive for when using a transistor as a switch. If your supply is 7V and the motor is dropping 5.5V when Q1 is on then Q1 not saturated. Q1 is dropping 1.5V which is 7.5 times higher than 0.2V. Since you're concerned about not exceeding 6V across the motor you would be well advised to use a 5V source and lower the base resistor until the collector emitter drop is closer to 0.2V.

As far as theory of operation is concerned I'll leave that to Alec, since he posted that design. I will tell you this though. Since the advent of the 555 end users have contrived circuit designs that the designers didn't include in their datasheets and possibly never conceived. The first time I saw the design that Alec posted I thought the designer made a mistake. It's no mistake though. As you can see it doesn't employ use of the discharge pin but works fine. It's become quite (SOP) common.

Chris

 

[Alec_t]

Would someone please explain: 1) why no discharge 7 pin connection 2) how the 2 diodes connected to 20 (on), 100K (off), and trigger (2) and output (3) are all working together and 3) what the electrolytic capacitor is doing.

The 555 output stage includes push-pull transistors which can pull pin 3 (Out) up to within a volt or so of Vcc, or pull it down fairly close to Gnd. An additional internal transistor connected to pin7 (Dis) optionally assists the pull-down if it's necessary to get close to Gnd as quickly as possible (which implies a large discharge current). The output stage is driven by a latch. The latch is set when Trig drops below about 1/3 Vcc and is reset when Thrs rises above about 2/3 Vcc.
The electrolytic cap is the timing capacitor and is initially discharged, so Trig is initially low, the latch is set and pin 3 goes high. The cap now charges via the 20k and D1. When the cap voltage reaches 2/3 Vcc the Thrs pin recognises this and resets the latch. Pin 3 (Out) goes low and the cap now discharges through pin 3 via D2 and the 100k. When the cap voltage reaches 1/3 Vcc the Trig pin recognises this and sets the latch again, thus starting another cycle.

 

[BobK]

If you use the discharge pin you cannot get duty cycles of 50% or less. If you use the output pin to do the discharge you can. That is is why it is done that way.

Bob

 

[CDRIVE]

Chris thanks for that tip; collector current is around 120 mA and base resistor current around 10 mA with 470 ohm resistor, so that seems like good saturation. I won't connect motor directly to 7v power because that will fry motor. There must be enough voltage drop with diodes and NPN that I'm getting around 5.5v on output.

Thanks.

p1ne, please elaborate on the Diode voltage drop you're referring to. The protection diode across the motor doesn't contribute to voltage drop. The only other diodes in the Alec's circuit are used for charge and discharge path for the timing cap. They have no effect on voltage drop in the motor leg of the circuit. If the supply voltage is stable then the only component that will effect voltage supplied to the motor is the transistor.

Chris

 

[CDRIVE]

This is an addendum to my last post. I just ran some DC Transfer curves for a 2N2222 with a load current of 120mA and something isn't adding up at your end. The only explanation that I can extrapolate from this is that your power source is not stable. What exactly is supplying your 7V, which is an odd voltage.

Chris

 

[HellasTechn]

Hellas, I've got an Arduino and I've read about similar application using Arduino triggering NPN. First, I want to learn something about PWM using discrete components.

Good practice !

 

[CDRIVE]

Yes, that would be good practice but a 555 is hardly a discrete component.

Chris

 

[p1ne]

Thanks Alec for your explanation about the cycle. If low trig 2 sets the latch and output 3 goes high, then this is an active-low circuit.

Following your post, I see how resistance from 100K creates longer "off" time as cap is being discharged, and looking at pos/neg direction of diodes it's clear to see how current passes through or is getting blocked for on/off times.

I notice that disconnecting reset pin 4/V+ pin 8 isn't making a difference in the output/motor. I'm reading that the reset pin overrides trig. So what function is the reset pin doing here?

Also thanks Bob for explanation about discharge vs. output with < 50% of duty cycle (more "off" time).

 

[Alec_t]

I notice that disconnecting reset pin 4/V+ pin 8 isn't making a difference in the output/motor. I'm reading that the reset pin overrides trig. So what function is the reset pin doing here?

Simply disconnecting tpin 4 won't reset the circuit. It needs to be pulled down to < ~0.7V for that. Power-up reset is often used with 555 circuits to ensure they start up in a known state. It's not used in your circuit.