REPORT ON EHARVEST SUMMER WORK

1. Frequency response of electrets condenser microphone :

An electret microphone takes a biasing voltage, supplied by a DC source, to work as a microphone to pick-up acoustic waves. The drainage current is quite small, but the output due to acoustic energy is even smaller in magnitude.

The circuit used to analyse the characteristics of the electrets microphone, used a LM324 single power op-amp.

FREQUENCY RESPONSE OF LM324: Input given from FG

Graph: Gain vs frequency in Hz.

The Pre-amp circuit used:

The frequency response after signal amplification by LM324 is given below: Input from MIC

Graph: Gain of pre-amp vs frequency (in kHz)

2. Drainage current into Electret condenser microphone :

A current output of 245uA, across a 2.2k-ohms.

3. Control signal for MOSFET bridge using LM339:

Circuit used:

Requirements: Vcc >= 1.25V , Vee = 0V

Complimentary set of outputs from comparators:

Here CH-2 is the incoming signal form piezo and CH-1 is the comparator output.

Note:

1. Control signal synchronised with the incoming signal.2. The circuit worked for the whole audio range (0 Hz =<f<= 50 kHz)

4. Piezoelectric buzzer internal circuit schematic:

This circuit gives a sinusoidal acoustic output for a constant DC Vcc. We measured the outputs at the middle and inner points of the ceramic piezo element w.r.t the grounded outer metal disc. The two voltages were found to be 1800 of phase. N.B.: This is a self-driven circuit, with the inner connection providing feedback for oscillations.

5. Study of ceramic piezoelectric element:

The piezoelectric element is connected in self-drive configuration, for the above given circuit.

To test the mechanical to electrical conversion of piezoelectric element, we use the External drive circuit wiring.

6. Rectification of piezoelectric buzzer output (using the vibration from massaging machine):

Resistor load of 212k ohms (including 1 mega ohms of CRO ) after the diode bridge rectifier, as piezo has 220 k ohms resistance at 34 Hz.

Without any capacitor in parallel with load resistance:

when we have a 1 micro Farad capacitor in parallel with the load resistor :

Screenshots: output across load in CH-2 (below), Output across piezo in CH-1 (above)

7. Current output from piezoelectric element:

Exciting the piezo element using cell-phone vibrations.Current = 453 mV/500 k =0.9 u A (p-p)

Then we attempted to couple acoustic waves with the piezo buzzer. The output voltages were still pretty impressive:

Current= 0.122 uAf= 440 HZ

f= 2kHz

8. Internal resistance of ceramic piezoelectric element as a function of frequency :

Note: Vibrational massage machine oscillated at 34Hz.

Resistance (in ohms) vs frequency (in Hz)

Resistance (in ohms) vs frequency (in Hz)

Conclusion: At low frequencies the piezo element acts as a current source, since it has very high internal resistance. At very high frequencies of >= 9kHz the piezo has comparatively low resistances, of the order of 900ohms.

Testing needs to be done as to the implication of piezo acting as a current source.

9. MOSFETs as switch: Study of drain to source drop:

Circuit used for p MOS:

IRF 640 B (n-MOS) : requirement : V_GS >= 1.6 V, Supply >= 0.2 V

Drop in switch (V_DS) = 187.5 mV

Supply=2.56V, current= (0.2-0.1875) /1 k = 12.5 uA ---can be as low as this (for Vcc as small as 2 V)

IRF9530 (p-MOS) : requirement : V_GS >= 1.5 V, Supply >= 1. 74 v

Drop in switch (V_DS)= 2.26-1.875= 391 mV (too high) --------Possible reason : Since they are power mosfetsSupply= 2.26 V

10. Comparison of diode and MOSFET bridge rectifier:

MOSFET:Input from FG ; DN rampbase wave SINEAM freq 100Hz

Diode:

Conclusion: Power MOSFET bridge gave almost the same output as the diode bridge though they require control signal for their gates, unlike the diode bridge.NEEDED: low power MOSFETS, MOSFET arrays like ALD series

11. A mount for studying vibrations on the piezoelectric element, by mounting 4 of them, asymmetrically on an epoxy raisin board, using araldite adhesive was made.

12. Shaker used at Mech. Dept:Observations:The piezo device setup resonates at 53Hz (2 in series-700mV), 54Hz (single – 280mV), 55Hz (4 in series – 800mV).According to the voltage output, we concluded that the piezo elements were vibrating out of phase, because of asymmetrical mounting.

13. An experimental setup for verifying the phase difference between the o/p of 2 piezo devices excited at the same time:

Subtracting o/p s of two piezos:

Screenshot:

CH-1 & CH-2 show outputs from two different piezos.

The middle waveform is the subtraction of waveforms generated from 2 piezo elements and clearly shows that they are not in same phase.

14. Investigation of DC offset in output from piezo element:

No vertical stress given : (this waveform confirmed the o/p to be AC and there is no DC offset)

Vertical stress given: (DC offset)

Conclusion: the DC offset present in earlier experiments were a result of stress applied by hand to hold it on the vibrating platform. Without any stress, it gives out a perfect AC output. So, a full bridge rectifier is needed.

15. A overview of button cell specifications as well as costs is available.16. A overview of required Schottky diodes and Step up DC-DC converters has been made.

Schottky diodes : UC1611, UC3611.

Step up DC-DC converter : MAX1674-1676, TPS61040.

17. Boost converter, using a 555 and an salvaged inductor:

Screenshot:

CH-2 (below) is input from diode bridge rectifier fed from piezo device.CH-1 (above) is boosted output.

By changing the duty cycle (D), we are able to change the voltage of output.A measurement of output current needs to be done.

18. Buck converter :

We used 555 instead of the 8038+comparator.

A voltage multiplication of .16 times is obtained.

Input = 13. 7 V (DC) , output= 2.228 V (DC),

19. Single polarity input to bipolar output (attempted):

Using input as a sinusoid wave we got the output as shown in the screenshot.