Download This handbell design uses four circuit configurations to drive the

This handbell design uses four circuit configurations to drive the speaker including: a tilt
sensor with amplified current, a linear voltage regulator, an op-amp relaxation oscillator, and a
bridge tied load.
The circuit configuration for the tilt sensor is shown in Figure 1.
Figure 1: Circuit configuration for a tilt sensor with amplified current
The two connector inputs labeled LED and GND are used for providing current to the tilt
sensor’s LED. A 470Ω resistor is used to limit the current. The collector terminals of both
phototransistors are tied together inside the tilt sensor. An external 3.3kΩ is used to limit the
current. When the tilt sensor is off, there is a leakage current of 11μA flowing through the
emitter E1. When the tilt sensor is on, there is a current of about 506μA flowing through the
same emitter. This current is not enough to do useful work. However, it can be amplified by
using another 2N4401 transistor as shown in Figure 1. The 10kΩ prevents leakage flowing from
tilt sensor. With this configuration, the current was amplified to approximately 14mA.
Figure 2 shows the circuit configuration for a linear voltage regulator.
Figure 2: Circuit configuration for a linear voltage regulator
The output voltage is observed and the voltage vce across the 2N4410 transistor is
adjusted such that vout is constant. The 78L05 voltage regulator maintains vout = 5V. In this case,
two capacitors each of 0.1μF could be added on the two legs of the 78L05 device to prevent
leakage current when the circuit is in its OFF state.
The circuit configuration for an op-amp relaxation oscillator is shown in Figure 3.
R
VCC
R2
R1
V+
+
V-
+
VO
TLV2371
-
R1
C
FIG. 1
Figure 3: Circuit configuration for an op-amp relaxation oscillator
The specification for this design necessitates that the circuit be powered from a single 9V
battery source. This requires an op-amp that is designed to work from a single supply. The opamp chosen for this application is the TLV2371. In addition to working from a single supply, this
op-amp also features rail to rail operation. This means that both the output and input can have a
voltage swing that goes from 0V to VCC with no offset voltage at both input and output.
Observe that the resistors labeled, R1, are equal in value and therefore form a voltage divider
that biases the positive input to the op amp at half the supply voltage. This implies that the output
voltage, VO, has an average voltage that is also at half the supply voltage. The output signal is a
square wave that will then have a peak to peak voltage swing from ground, 0(V), to saturation of
VCC. The output waveform is shown in Figure 4.
Figure 4: Output waveform of an op-amp relaxation oscillator
It can be shown that the period of this output waveform is:

R 
T  2RC  ln 1  1 
R2 

In other words, the period is determined by four parameters R, C, R1 and R2. The
designer has to pick values for these parameters to get a desired frequency from the oscillator. In
this case, the handbell is required to be frequency adjustable. This could be achieved by making
resistor in the oscillator a potentiometer. The value of the potentiometer can be derived using the
fact that the frequency range is between 98Hz and 880Hz.
Let R1  2.2k, R2  3.3k, C  0.1F .
 2.2  1
Then, T  2 R0.1  10 6   ln 1 
if and only if R  100k

 3.3  98
1
 2.2 
Similarly, T  2 R0.1  10 6   ln 1 
if and only if R  10k

 3.3  880
Thus, a 100kΩ potentiometer with 25 turn would be appropriate for this application.
This circuit configuration was simulated by using LTSpice as shown in Figure 5. The
value of resistor R were set to be at both 10kΩ and 100kΩ to simulate the frequency. The
simulation results are shown in Figures 6 and 7.
Figure 5: An op-amp relaxation oscillator built in LTSpice.
Figure 6: Output waveform of the oscillator using R = 10kΩ
Figure 7: Output waveform of the oscillator using R = 100kΩ
It can be observed that the output voltage swing is between 0V and 5V as expected. The
frequency for both cases can be calculated approximately as followings.
1
 909 Hz
7.8  6.7
1
f100k 
 96 Hz
2(14.6  9.4)
Finally, the circuit configuration for a bridge tied load whose input voltage is the output
f10k 
voltage of the oscillator is shown in Figure 8.
Figure 8: A bridge tied load connected to an oscillator
Notice from Figure 8 that the non-inverting op-amp is actually a buffer. Since the input
voltage vin and VCC are both 5V. The maximum gain for this circuit is therefore equal to 1. In
addition to that, there would be some offset voltage since this is not a rail to rail op-amp.
On the other hand, the resistor values of the in inverting op-amp were derived such that
the value of gain K is 1.
R5 R8  R9 
R
V  8 vin (t ) .
R8 ( R5  R6 )
R9
In this case, it is desired that v (t )  vin (t )  5 so that when vin(t) = 0V, v-(t) = 5V, and
when vin(t) = 5V, v-(t) = 0V.
R
Thus, 9  1  R8  R9
R8
5R5 R8  R9 
and
 5  R5  R6
R1 ( R5  R6 )
Therefore, R8 and R9 can be chosen to be 22kΩ to make sure that there is not too much
It can be shown that for an inverting op-amp, v (t ) 
current drawn from the circuit. Also, R5 and R6 can be chosen to be 2.2kΩ.
The simulation result for the circuit in Figure 8 is shown in Figure 9. It can be observed
that the average value of output voltage driving the speaker (vout(t) = v+(t) - v-(t)) is 0 as
expected. Also, the volume of the speaker can be adjusted by adjusting the power delivered to
the speaker. This can be achieved by connecting a 10Ω potentiometer to the output of the
inverting op-amp.
Figure 9: Simulation result for oscillator – bridge tied load cicuit
Figure 10 shows the final assembly of the hand bell and Table 1 gives the final bill of materials
and costs of the projects circuit.
Figure 10: Final assembly of the PCB board with case, battery, and speaker
Table 1: Final bill of materials for PCB board construction
Part
PN2222 NPN Transistor 60V
0.6A
PN2907 PNP transistor 60V
0.6A
TLV2372 Dual RRIO op-amp
Distributor
Distributor
Part #
Per-Unit
Cost
Digi-Key
PN2222BU-ND
$0.11
4
$0.44
Digi-Key
Digi-Key
$0.12
$1.30
4
4
$0.48
$5.20
TCA0372 Dual power op-amp
78L05 5V 100mA voltage
regulator (30V)
0.1uF ceramic capacitor 50V
10 ohm 25-turn potentiometer
100k 25-turn potentiometer
330 Ohm
470 Ohm
2.2k Ohm
3.3k Ohm
10k Ohm
22k Ohm
9V battery snap connector
Digi-Key
PN2907ABUFS-ND
296-12219-5-ND
TCA0372DP1GOSND
$1.25
4
$5.00
LM78L05ACZFS-ND
BC1160CT-ND
490-2872-ND
490-2876-ND
330QBK-ND
470QBK-ND
2.2KQBK-ND
3.3KQBK-ND
10KQBK-ND
22KQBK-ND
BS6I-HD-ND
$0.20
$0.07
$0.85
$0.85
$0.05
$0.05
$0.05
$0.05
$0.05
$0.05
0.33
4
12
4
4
4
4
16
8
8
8
4
$0.80
$0.79
$3.40
$3.40
$0.22
$0.22
$0.86
$0.43
$0.43
$0.43
$1.32
SK-286
425-1961-5-ND
$1.20
$2.45
4
4
$4.80
$9.80
Total
Cost
$38.02
8 ohm 2.25” 1/2W speaker
Tilt sensor
Digi-Key
Digi-Key
Digi-Key
Digi-Key
Digi-Key
Digi-Key
Digi-Key
Digi-Key
Digi-Key
Digi-Key
Digi-Key
All
Electronics
Digi-Key
Qty.
Total
Price