# Electrical Engineering/Speaker schematic on MTS51

Question

schematic1

schematic2
QUESTION: Hi!
I'm doing a little project on MTS51 Microcomputer trainer using speaker. So I made up schematic of it. Is it looks acceptable?

Probably next part of this task will be most difficult and tricky for me. I have to prove, why exactly these components with specific values are given.
What would be most appropriate way to start?
Speaking about the 74LS14.. it's used to clean up the contact bounce on the signal edges, restores the rise time and cleans signal. Is it something like that?
To find out possible current in the speaker, I have to divade voltage with speaker resistance I = U/R? I have to use voltage of point after inverter 74LS14?

ANSWER: The speaker has direct current passing through the voice coil which will give it a biased cone positioning. Further, depending on the Vcc voltage the speaker could have conditions where the maximum current (0.8 amp).  The speaker may get burned out.

It is not a good idea to have dc on the speaker.  And you must have some emitter resistance to keep the transistor from over heating or over voltage on the collector.

The rest of the circuit looks okey to me.  You just need to make a table of each component and fill the grid with its chief function for the circuit.

Since I don't know your application and power supply voltages (vcc) I can't give you much more advice.

Let me know if you need more.

PS:  Study the spec notes on the MTS51 and 74ls14 and it will guide you through some of the necessities.

---------- FOLLOW-UP ----------

QUESTION: I don't know model of the speaker used there, so I don't know Ic and speaker resistance, but as I know max possible current values of D1, T1...

I can guess, that speaker current is lower than 300mA. For example, diode 1N4148 maximum forward current rating = 300mA and for transistor

2N2222 maximum current = 0.8 A. So load (speaker) current should be something about 200mA to be safe. Let's imagine I use, for example, this

speaker: http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_2095234_-1 (P = 0.2W and R = 8Ohms). Using P = I^2 *

R, I can calculate max current for speaker, in this case it would be 0.15 A. Diode can stand it. Is it right?

Vcc = 5 V. It's microcontroller P89C51RD+.
Speaking about 74LS14, it takes a digital input and outputs the opposite and it helps to generate square wave signal. "High" becomes "low", and

"low" becomes "high". I read that "high" signal should be anything about 3.2V. After signal goes trough the invertor and resistor R1, we get base

current. Or not?
It would be good to prove why R1 is 820 Ohms or so (because we don't know real speaker parameters, but we can base of my chosen one

speker specification). What would be best the way to start calculations? Maybe you can offer any hints about the formulas?

You sad about the emitter resistance, but it wasn't included in schematic in orginal MTS book (my previous post: Image: schematic1), I guess

these currents are small enough to be safe. And as it wasn't shown in book, not so sure, that I should add some extra elements there.

When I checked my program code on MTS51, all was working good. By the way, here's the task: Speaker sounds a 2kHz (Delay 0.25ms to

generate frequency) tone 1 second and then keeps silence 1 second (Delay 1 s).

That's OK, but as you can see, the toughest part for me is to describe elements and whole circuit..

Look; your comments are all over the place and it is apparent that you do not understand basic circuit theory or Ohm's law for components and circuits.

If the transistor get enough drive and is turned on to saturation the entire power supply voltage (which you still don't give) will be across the speaker.  If the speaker is 8 ohms as in one of your comments and your supply was 16v then you could get a current of 2 amps through the speaker and transistor which  would be lethal to each of them.

Without more details of the design it will be difficult to advise you further.  Sorry.

For example; must have the value of Vcc in order to determine the current in the speaker and the transistor.  If you are getting a pulse of sufficient amplitude into the base of the transistor it will switch it to saturation and put a pulse of maximum current through the speaker  and the collector circuit of the transistor.

That's for starters.

Electrical Engineering

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#### cleggsan

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