Electronics and Physical Interfaces

1
Electronics and Physical Interfaces

• Speaker Interface
• Electronics Review
• 8051 I/O

2
Speaker Interface

• Add waveforms to get multiple tones (think
  current through speaker, not voltage)

Note that lower frequency is smoother for a given
sample rate
3
Ideal Solution

• Digital to Analog Converter

We can probably find a chip for this
8051
8
DAC
AMP (V to I)
Voltage signal
Speaker cares about current, not voltage What is
algorithm to superimpose 1KHz tone with 500Hz
tone With a sampling rate of 10KHz
4
Synthesizer Algorithm

• Let sin be a look up table with 256 entries (1
  complete cycle)
• Every .1ms (10KHz)
• P2 sint1 sint2 sint3
• For 1KHz, t1 256/10 is this hard? how do we
  implement this?
• For 500Hz, t2 256/20
• At 8-bit resolution we can vary output from 0 to
  255. Hi frequencies are smoother
• Can Compute arbitrary waveforms (not just tone
  summations)
• This is probably quite doable with our 8051, but
  we will take a simpler approach (maybe some of
  you may take the last week to improve the
  acoustics)

255
128
0
Computer each sample through
5
Using single bit tones

• Two tones generated by single bit outputs.
• Use current summation externally.
• Note high frequencies look more sinusoidal

6
Our Version

• Objective convert number of bits to current

5V
8051
tone1
AMP (B to I)
tone2
tone3
7
One Idea
5V
8051
R?
tone1
8ohms
tone2
Vs
tone3
Constraints 5/(8R)lt10mA 5/(8R/3) lt 26mA
Does this sum? (I Bx) What must R be to
protect the processor? What is the worst case (1,
2, or 3 bits)? How much power are we putting
through the speaker in the worst case?
No quite. Vs decreases with increasing B. So each
Ir decreases with increasing B
3 bits is worse (Higher R constraint)
(26mA)2 8 5.4mA
8
Another Idea

• Use a current amplifier (PNP Transistor)
• Ice ?Ib (assume ?10)
• Assume Vbe 0 (can be up to .7V)
• Assume tone1 0V
• Determine Ib, Ice
• Determine Power dissipated by speaker

5V
8ohms
Ib
8051
e
Ib 11Is Is 5/8 Is 625mA Ib 57mA Ic
568mA Violates speaker, 8051, Transistor
constraints Ps (.6252) 8 3W (overpowers
.2W speaker)
tone1
b
?Ib
c
9
How do we fix this problem

• How much current can we safely put through a .2W
  speaker?
• Imax (.2/8).5 158mA
• From data sheet for transistor, notice that Ice
  50mA if Ib 5mA, and it gives us values for Vce
  and Vbe for that case. Is this a good on
  operating point for us? (Other operating points
  can be found on graphs, experimentation)
• Its perfect if we use one for each tone for a
  total of about 50mA
• If we limit Ib then we limit Ice too. What size
  resistor should we put between 8051 port and the
  BASE to bias the transistor properly. Heavy use
  of KCL and Ohms law. See next page

10
Final Circuit Design
Size R to match your speaker and to Stay within
the current limitations of the Processor. From
DataSheet (On Sat.)
5V
Ib 5mA Ic 50mA Vbe .65V (min) Vr (5
(50mA8) - .65) R Vr/5mA 790ohms
8ohms
8051
R?
tone1
tone2
tone3
11
Resistor

• Review of basic Electronics
• Capacitors
• Inductors
• Bipolar Transistors
• MOS transistors
• Review of 8051 I/O configuration