555 monostable timer, pulsed LED and buzzer motor

Thread Starter

dMEK

Joined Apr 4, 2019
16
I need this circuit to pulse an LED while running a buzzer motor for an adjustable period

RV1 and C1 should give me 1 - 5s of pulse, while RV2 and C2 should adjust the LED ramp times

The LED will be a strip of up to 10 white LEDs

Will the 555 drive these transistors, or do I need a third transistor to drive both of them, or would mosfets work more effectively?

Also, does pin 5 need a 10 nF cap on it (and if so, why?)

Thanks



 

AlbertHall

Joined Jun 4, 2014
10,047
It would be better if you put the motor and D2 between the collector of Q2 and the supply. Connect the emitter of Q2 to ground. You might also need a resistor from Q2 base to ground to make sure the motor turns off completely.

How much voltage and current does the LED strip need?
 

ElectricSpidey

Joined Dec 2, 2017
1,175
A bipolar 555 can provide aprox. 200mA source current.

Some 555 circuits will run ok without the CV cap, but I would use it, especially if you are running a motor.

You might get a better “ramp” on the LEDs if you reconfigure Q1 as a “follower” that is place the LEDs and the resistor on the emitter instead of the collector, this way the voltage on the LED will “follow” the RC network instead of acting more like a switch.

Also move the motor to the collector of Q2 as already pointed out.
 
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Thread Starter

dMEK

Joined Apr 4, 2019
16
Excellent

Thanks for the advice

The LED strip is parallel, 5V. 10 white LEDs I think will be ~600mA of current

I haven't settled on the best motor for the application as yet
 

Thread Starter

dMEK

Joined Apr 4, 2019
16
So this is the updated schematic

How does one determine a good value for R4 on the motor? (I have a few different motors to test before deciding on the final design)
a couple of the motors are 3V so will have to include an LM voltage regulator for its supply

D



upload_2019-4-9_7-40-28.png
 

djsfantasi

Joined Apr 11, 2010
6,510
So this is the updated schematic

How does one determine a good value for R4 on the motor? (I have a few different motors to test before deciding on the final design)
a couple of the motors are 3V so will have to include an LM voltage regulator for It’s supply
Why do you need R4 at all?

I notice that you haven’t added a resistor from the base of Q2 to ground as suggested. Is this why R4 was added between the emitter and ground? If so, it’s misplaced.
 

eetech00

Joined Jun 8, 2013
1,938
So this is the updated schematic

How does one determine a good value for R4 on the motor? (I have a few different motors to test before deciding on the final design)
a couple of the motors are 3V so will have to include an LM voltage regulator for its supply

D
View attachment 174531
Move R2 and D1 to the collector side of Q1. If you place it on the emitter side, it will change the base current with the load current, which will change the gain setting of transistor Q1.
Remove R4 (unless you want a current limiter, in which case it should be small). A resistor from the base of Q2 to ground will ensure Q2 is turned off.
 

ElectricSpidey

Joined Dec 2, 2017
1,175
eetech00 I’m a little confused, in all my experience over the years creating “ramped” outputs to light LEDs the emitter follower config is superior to the common emitter.

Also if the OP wants to go back to placing the load on the collector they should place another resistor in series with the base, because as is RV2 can be adjusted to a short.
 

AnalogKid

Joined Aug 1, 2013
8,489
In both #1 and #7 schematics, the LEDs will ramp on and off. They will change more quickly in #1, but they will not "snap" on.

ak
 

eetech00

Joined Jun 8, 2013
1,938
eetech00 I’m a little confused, in all my experience over the years creating “ramped” outputs to light LEDs the emitter follower config is superior to the common emitter.
Superior in what way?

Also if the OP wants to go back to placing the load on the collector they should place another resistor in series with the base, because as is RV2 can be adjusted to a short.
yes...missed that.

eT
 

AnalogKid

Joined Aug 1, 2013
8,489
Superior in what way?
An emitter follower has a voltage gain of 1, so whatever (shape, timing, slope, ramp, whatever) signal is on the base is reproduced (-ish) at the emitter. OTOH, a common emitter saturated switch has a gain of 100-ish through its linear region between full off and full on. This compresses the shape of the input signal, making the slope much more steep. For the same slope waveform into both circuits, you will see a much more gradual change in light output with the emitter follower circuit, while the LEDs will appear to have more of a snap from dark to light with the saturated switch.

If If If you want to see the LEDs fade on and off, then the emitter follower is the superior circuit.

ak
 

eetech00

Joined Jun 8, 2013
1,938
An emitter follower has a voltage gain of 1, so whatever (shape, timing, slope, ramp, whatever) signal is on the base is reproduced (-ish) at the emitter. OTOH, a common emitter saturated switch has a gain of 100-ish through its linear region between full off and full on. This compresses the shape of the input signal, making the slope much more steep. For the same slope waveform into both circuits, you will see a much more gradual change in light output with the emitter follower circuit, while the LEDs will appear to have more of a snap from dark to light with the saturated switch.

If If If you want to see the LEDs fade on and off, then the emitter follower is the superior circuit.

ak
Thanks
 

Thread Starter

dMEK

Joined Apr 4, 2019
16
Version 3 then

I did misread the purpose of R4 and misplace it

With the emitter follower circuit for fading the LEDs, do I still need R5 in series?

Obviously I will have to work our the value for R3 once I've settled on the motor I'll be using - I presume if the combined base current for both transistors exceeds 200mA, then the solution is to add another transistor to switch them both?

Thanks heaps

D

upload_2019-4-10_15-50-40.png
 

AnalogKid

Joined Aug 1, 2013
8,489
For the schematic in #16 -

The left side of R3 should connect to U1 pin 3 directly.

Now that Q1 is back to being an emitter follower, there is no need for R5.

You DO need a resistor between U1 pin 7 and RV1 pin 2 OR one between RV1 pin 3 and Vcc. 1K in either place is enough. The discharge function is a single open collector transistor to GND. When the pot is fully CCW, Discharge will try to short Vcc to GND. Poof.

If you don't already own the 10K pot, I'd change RV1 to 100K and C1 to 47 uF. Capacitor leakage current will be much less, for more accurate (and stable) timing.

ak
 
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AnalogKid

Joined Aug 1, 2013
8,489
But then at one end of RV2, C2 (220uF) will be connected directly to the '555 output pin.
True, but there's nothing automatically wrong with that since R5 is not needed to protect the Q1 base.

If the intent of R5 is to set a minimum ramp time, 220 ohms is kinda small. That's a time constant of around 50 ms*, and the LEDs go from full dark to full bright in only a portion of that.

* The time constant calculates out to 48.4 ms, but Q1's base current steals away some of the charging current, increasing the ramp time.

ak
 
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Thread Starter

dMEK

Joined Apr 4, 2019
16
R5 was just in case of a short through RV2
RV2 is to tune the ramp time, realistically it will always be larger than R5 - I figure a ramp of 0.25*pulse time, so probably around 300 - 500ms
If I keep R5, I should be able to ditch R3, correct?
 
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