Working with an RC timer and a transitor to power a high current device

[Mark65]

Hello to the forum. I was wondering if someone could help me understand the problem described below.

I'm working through some experiments with electronic components and have hit a hurdle trying to power a piezo buzzer via an rc timer/transistor combination. I've attached a picture of the circuit to this email and was wondering if you could help me understand how to meet my goal.

Alone the RC timer (r1 and c1) will charge the capacitor to 5v as you would expect but once I connect the transistor the voltage drop across c1 never exceeds .7v. I figured that this is because the current leaks through the transistor's base/emitter as the transistor becomes a switch. I'd appreciate it if you could confirm this.

Okay, the RC timer works and once the base reaches .7v the collector/emitter connects and I can power an LED.

All good, but I then I replace the LED with a 12v peizo buzzer and it doesn't make a sound. Reading the current between the LED/buzzer and collector I've recorded 1.4mA passing through the buzzer and 6.8mA through the LED.

The buzzer when connected directly to the 5v power supply makes a noise and I've noticed it draws 7.9mA and when connected to 3.3v draws around 4mA and still makes a noise.

I've confirmed the polarity of the buzzer and can see that the collector/emitter can provide at least 6.8mA but cannot understand why the current drops to 1.4mA when I introduce the buzzer.

What is the solution to this problem?

 

[Colin Mitchell]

See the 120k resistor . . . . . . the transistor effectively reduces this value by 100 (the gain of the transistor). So, the transistor becomes equal to 1.2k.
Put a 1k2 in place of the transistor and see how loud the piezo is.

 

[Mark65]

Colin, I'm sorry for the slow response. I'm just recovering from a serious cold. I do appreciate your efforts.
Indeed, you are correct that having just a 1k2 resistor means the piezo buzzer doesn't make a sound.
Electronics! I shouldn't be surprised.
I've started reading about 'the gain of the transistor' but if you know of a great blog/page/paper it'd be appreciated.
I was expecting the line going to the collector was quite separate to the rest of the circuit.
Being that R1(120K) is part of the RC timer, what are my options?
Does the R2(1K) resistor play apart in this.
Your help is most appreciated.
Thanks,
Mark

 

[Colin Mitchell]

What do you want to do?

 

[Mark65]

In this curcuit, how can i make the peizo buzzer 'buzz'?
Thanks

 

[AnalogKid]

You have a couple of options. The 120K resistor is starving the base of the current it needs to turn on the transistor enough to sink enough current to make the piezo audible. (A long a messy sentence) but when you decrease the 120 K, the time delay will decrease. If you decrease the 120K to 10K and increase the 470 uF to 4700 uF, the delay will be approximately the same but the transistor will be able to sink more current, probably enough for the piezo. Note that the beeper will no come on crusply. It's volume will increase slowly enough to notice. When you disconnect the power source, the piezo volume will decrease slowly.

Another option is to change to a Darlington transistor or add another 2N3904 to the one you have to make one. With this change, the transistor will need much less current to turn on, and the beeper will turn on and off more quickly.

Is there a specific time delay you need for this circuit?

ak

 

[Mark65]

AnalogKid, thanks for the help.
There's no great focus on the time delay I'm just using this to demonstrate how to power a high current device, the peizo, from a low power switch, the base of the transistor.
I'm looking to use what i learn here in working with PIC programming.
Thanks,

 

[AnalogKid]

If that's the intent, why the large cap and large resistor, a clear sign of a delay requirement?

ak

 

[hevans1944]

AnalogKid, thanks for the help.
There's no great focus on the time delay I'm just using this to demonstrate how to power a high current device, the peizo, from a low power switch, the base of the transistor.
I'm looking to use what i learn here in working with PIC programming.
Thanks,

If you discard R1 and C1 and connect the now free end of R2 directly to a PIC output port, with the emitter of the transistor connected to common (Vss) and your LED or piezo buzzer or whatever connected between +5 VDC (Vdd) and the transistor collector, you should be able to turn things on or off by writing to the output port. The problem with your test setup is the high resistance of R1 preventing the transistor from turning fully on and saturating. If you get rid of R1 and C1, you can also turn the transistor on by connecting the free end of R2 to +5 V or Vdd. You can turn it off by connecting the free end of R2 to "ground" or common or Vss.

BTW, a piezo transducer is not generally considered to be a high-current device. Motors and incandescent lamp loads and 10 W LEDs are generally considered to be high-current devices, requiring something a bit beefier than a 2N3904 transistor. Check the 2N3904 datasheet for current limitations. It should be either driven to collector saturation or driven off when controlled by a PIC. Have fun!

 

[Mark65]

If that's the intent, why the large cap and large resistor, a clear sign of a delay requirement?

ak

Hello all, i started experimenting with electronics after reading about rc timers, i made an led light up and then moved onto the peizo buzzer. I've programmed PIC but found that my electronics knowledge was my weak point. Hence the simple designs and questions. The large cap and resistor was just so that i could show my family at least something measurable and impresive.

 

[Mark65]

If you discard R1 and C1 and connect the now free end of R2 directly to a PIC output port, with the emitter of the transistor connected to common (Vss) and your LED or piezo buzzer or whatever connected between +5 VDC (Vdd) and the transistor collector, you should be able to turn things on or off by writing to the output port. The problem with your test setup is the high resistance of R1 preventing the transistor from turning fully on and saturating. If you get rid of R1 and C1, you can also turn the transistor on by connecting the free end of R2 to +5 V or Vdd. You can turn it off by connecting the free end of R2 to "ground" or common or Vss.

BTW, a piezo transducer is not generally considered to be a high-current device. Motors and incandescent lamp loads and 10 W LEDs are generally considered to be high-current devices, requiring something a bit beefier than a 2N3904 transistor. Check the 2N3904 datasheet for current limitations. It should be either driven to collector saturation or driven off when controlled by a PIC. Have fun!

hevans1944, thanks for this.

 

[hevans1944]

Hello all, i started experimenting with electronics after reading about rc timers, i made an led light up and then moved onto the peizo buzzer. I've programmed PIC but found that my electronics knowledge was my weak point. Hence the simple designs and questions. The large cap and resistor was just so that i could show my family at least something measurable and impresive.

If you can learn to program a PIC (it has taken me several months to learn how to do that), your should be able to blow the socks off your family with some pretty simple external "stuff" because all the magic will be in your firmware. I would go for some LED "chaser" light displays first. This is three or more LEDs that light sequentially, with perhaps four or more groups to give the appearance of a moving light display. Maybe your software could "read" an external switch to change the direction and perhaps a potentiometer you can "read" in software could be used to change the chase speed. With a PIC you don't need RC timers. Either use the built-in timer or write program delay loops, nested if necessary, to generate time delays. Sounds to me like you are on your way to having a lot of fun!

 

[Minder]

If your intention is to get into Pic and use it to control high(er) current devices I suggest you look at Logic level Mosfets, these do not load up the Micro like a bi-polar transistor does and also in many cases has a much lower on resistance, they are similar 3 terminal devices and cost no more so it is an easy transition.
A handy one to start with is the 2n7000, one of the first produced for integrating TTL & CMOS IC's to power devices.
M.