Issue
148 November 2002
Ultrasonic
Homing Device
Start
•
Reveiver • Ear
Mounting • The Transmitter
• Software • Options
• Sources and PDF
RECEIVER
The receiver circuit is
shown in Figure 2. The ultrasonic
receiver’s transducers have a narrow reception bandwidth
around their operating frequency of 40 kHz. After receiving
the transmitted acoustical energy, the receiver generates
an AC signal tens of millivolts in amplitude depending
on the transmitted signal strength and distance from
the transmitter to receiver. This small signal level
is amplified and converted to logic-level pulses by
two op-amps.
The input signal is capacitively
coupled through resistor R14 to the inverting input
of the first op-amp. Together, resistors R15 and R14
determine the gain by which the millivolt-level signal
is amplified using the following equation:
[1]
Because the gain is high,
the DC voltage must be blocked from the input signal
or it will be amplified, causing the output to run up
against the negative rail. This is accomplished with
capacitor C11. However, because C11 is a reactive component,
it has an associated impedance that must be added to
resistor R14’s value in the gain equation. The equation
for capacitive impedance is:
[2]
In Equation 2, f is the
ultrasonic frequency of 40 kHz, and ZC is measured in
ohms.
Using a value of 0.1 µF
for C11 shown in Figure 2, ZC = 40 W, which is much
less than R14 = 10 kW, so it can be ignored. The gain
is set at 75, which is less than the maximum allowable
gain at 40 kHz shown in the datasheet for the OPA2340
op-amp. Note that you can’t use just any op-amp; it
must be fast enough to provide the necessary gain at
the desired operating frequency. According to the OPA2340’s
datasheet, the typical maximum gain at 40 kHz is 100,
which exceeds the desired operating gain of 75, so this
op-amp will work.
Because a single-supply
op-amp is used, it’s necessary to create a virtual ground
at a level approximately half-way between power ground
and 5 VCC. Remembering that the voltage difference between
the inverting and non-inverting inputs to the op-amp
must be zero, the voltage applied to the non-inverting
input, in this case 2.5 V, is reflected at the inverting
input.
The DC output from the
op-amp will also be 2.5 V because C11 blocks the effect
of R14, effectively removing it from the circuit so
that the op-amp behaves as a simple voltage follower.
The AC signal is amplified, however, so the output of
the op-amp will then be the inverted and amplified 40-kHz
signal oscillating about virtual ground of 2.5 VDC.
The fact that the signal is inverted does not matter
because this is true for both ears’ signals, and they’re
only relative to each other.
The second op-amp after
the gain stage acts as a comparator. When the signal
on the non-inverting input exceeds the threshold level
established on the inverting input, the output goes
to the positive rail: 5 V. When the input signal is
less than the threshold, the output goes to the negative
rail: DC ground.
The threshold and virtual
ground are set with a common resistor divider network.
[1] From above, the DC output level of the gain stage
is set to virtual ground. The threshold level of the
comparator should be slightly higher than this. Then,
when the amplified AC signal level exceeds the threshold,
the comparator’s output goes high.
Equations 3 and 4 determine
the virtual ground (VGND) and threshold (VTH) relative
to the virtual ground.
[3]
[4]
Note that R11 determines
the threshold, with increasing values raising the threshold
level. Values that are too small will make the comparator
susceptible to noise. If they are too large, sensitivity
will decrease to an unusable level. Typical values range
from 100 to 2200 W.
The logic-level pulses
from each ear are monitored by two of the PIC microcontroller’s
general-purpose (GP) I/O bits. Two more I/O bits provide
output indicating which ear hears the ping first, if
both ears hear the ping at essentially the same time,
or if the signal fails to meet valid input conditions.
These can be used to control the wheel drive motor logic
directly or through another controller.
