Digital pedometer circuit

Max. range: 9.950 meters with tw1o digits

Slip it in pants` pocket for walking and jogging

R1,R3____22K 1/4W Resistor
R2________2M2 1/4W Resistor
R4________1M 1/4W Resistor
R5,R7,R8__4K7 1/4W Resistor
R6_______47R 1/4W Resistor
R9________1K 1/4W Resistor

C1_______47nF 63V Polyester Capacitor
C2______100nF 63V Polyester Capacitor
C3_______10nF 63V Polyester Capacitor
C4_______10΅F 25V Electrolytic Capacitor

D1_______Common-cathode 7-segment LED mini-display (Hundreds meters)
D2_______Common-cathode 7-segment LED mini-display (Kilometers)

IC1______4093 Quad 2 input Schmitt NAND Gate IC
IC2______4024 7 stage ripple counter IC
IC3,IC4__4026 Decade counter with decoded 7-segment display outputs IC

Q1,Q2___BC327 45V 800mA PNP Transistors

P1_______SPST Pushbutton (Reset)
P2_______SPST Pushbutton (Display)

SW1______SPST Mercury Switch, called also Tilt Switch
SW2______SPST Slider Switch (Sound on-off)
SW3______SPST Slider Switch (Power on-off)

BZ_______Piezo sounder

B1_______3V Battery (2 AA 1.5V Cells in series)

This circuit measures the distance covered during a walk. Hardware is located in a small box slipped in pants` pocket and the display is conceived in the following manner: the leftmost display D2 (the most significant digit) shows 0 to 9 Km. and its dot is always on to separate Km. from hm. The rightmost display D1 (the least significant digit) shows hundreds meters and its dot lights after every 50 meters of walking. A beeper (excludable), signals each count unit, which occurs every tw1o steps. A normal step is calculated to span approx. 78 centimeters, thus the LED signaling 50 meters lights after 64 steps or 32 mercury switch`s operations, the display indicates 100 meters after 128 steps and so on. For low battery consumption the display lights only on request, pushing P2. Accidental reset of the counters is avoided because to reset the circuit both pushbuttons must be operated together. Obviously this is not a precision meter, but its approximation`s degree was found good for this kind of device. In any case, the most critical thing to do is placement and sloping degree of the mercury switch inside the box.

IC1A & IC1B form a monostable multivibrator providing some degree of freedom from excessive bouncing of the mercury switch. Therefore a clean square pulse enters IC2 that divide by 64. Q2 lights the dot of D1 every 32 pulses counted by IC2. IC3 & IC4 divide by 10 each and drive the displays. P1 resets the counters and P2 enables the displays. IC1C generates an audio frequency square wave that is enabled for a short time at each monostable count. Q1 drives the piezo sounder and SW2 let you disable the beep.

Experiment with placement and sloping degree of mercury switch inside the box: this is very critical.
Try to obtain a pulse every tw1o walking steps. Listening to the beeper is extremely useful at this setting`s stage.
Trim R6 value to change beeper sound power.
Push P1 and P2 to reset.
This circuit is primarily intended for walking purposes. For jogging, further great care must be used with mercury switch placement to avoid undesired counts.
Current consumption with display disabled is negligible, therefore SW3 can be omitted.

Digital pedometer circuit

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U1a operates as a low-noise microphone preamp. Its gain is only about 3.9 because the high output impedance of the drain of the FET inside the electret microphone causes U1as effective input resistor to be about 12.2K. C2 has a fairly high value in order to pass very low frequency (about 20 to 30Hz) heartbeat sounds.

U1b operates as a low-noise Sallen and Key, Butterworth low-pass-filter with a cutoff frequency of about 103Hz. R7 and R8 provide a gain of about 1.6 and allow the use of equal values for C3 and C4 but still producing a sharp Butterworth response. The rolloff rate is 12dB/octave. C3 and C4 can be reduced to 4.7nF to increase the cutoff frequency to 1KHz to hear respiratory or mechanical (automobile engine) sounds.

The U4 circuit is optional and has a gain of 71 to drive the bi-colour LED.

U5 is a 1/4W power amplifier IC with built-in biasing and inputs that are referred to ground. It has a gain of 20. It can drive any type of headphones including low impedance (8 ohms) ones.

Part	Total Qty.	Description
R1	1	10K 1/4W Resistor
R2	1	2.2K 1/4W Resistor
R3, R9	0	Not used
R4	1	47K 1/4W Resistor
R5, R6, R7	3	33K 1/4W Resistor
R8	1	56K 1/4W Resistor
R10	1	4.7K 1/4W Resistor
R11	1	2.2K to 10K audio-taper (logarithmic) volume control
R12	1	330K 1/4W Resistor
R13, R15, R16	3	1K 1/4W Resistor
R14	1	3.9 Ohm 1/4W Resistor
C1, C8	2	470uF/16V Electrolytic Capacitor
C2	1	4.7uF/16V Electrolytic Capacitor
C3, C4	3	0.047uF/50V Metalized plastic-film Capacitor
C5	1	0.1uF/50V Ceramic disc Capacitor
C6, C7	2	1000uF/16V Electrolytic Capacitor
U1 U2, U3 U4 U5	1 0 1 1	TL072 Low-noise, dual opamp Not used 741 opamp LM386 1/4W power amp
MIC	1	tw1o-wire Electret Microphone
J1	1	1/8" Stereo Headphones Jack
LED	1	Red/green 2-wire LED
Batt1, Batt2	2	9V Alkaline Battery
SW	1	2-pole, single throw Power Switch
Misc.	1	Stethoscope head or jar lid, Rubber Sleeve for microphone.

Assemble the circuit using Veroboard (stripboard) or a PCB.

Use a shielded cable for the microphone as shown on the schematic.

Fasten the microphone to the stethoscope head with a rubber isolating sleeve or use a short piece of rubber tubing on its nipple. A thick jar lid can be used as a stethoscope head. The microphone must be spaced away from the skin but the stethoscope head must be pressed to the skin, sealing the microphone from background noises and avoiding acoustical feedback with your headphones.

The microphone/stethoscope head must not be moved while listening to heartbeats to avoid friction noises.

Protect your hearing. Keep the microphone away from your headphones to avoid acoustical feedback.