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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 0.1F . 2.2 1 Then, T 2 R0.1 10 6 ln 1 if and only if R 100k 3.3 98 1 2.2 Similarly, T 2 R0.1 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