Continue to Site

Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Register Log in

Problem with a heartbeat monitor circuit

Status
Not open for further replies.
Heartbeat Monitor

The HeMon circuit (look in Google) does not measure the saturation of blood with oxygen. It simply gives a pulse when a spurt of blood in the finger blocks light from the LED to the LDR. The display is the number of heartbeats per minute.
The patient does not touch the circuit so it does not need strict isolation.

The LM358 is a poor quality dual opamp that operates with a single supply voltage as low as 3V and has low supply current.
In the HeMon circuit it is a simple pulse amplifier so its noise does not matter.

The diodes in your new circuit protect the AD620 from blowing up from static electricity. If an electrical short circuit occurs then the diodes would blow up and the patient will be fried and electrocuted.
The output that feeds the computer must be isolated.
 

Re: Heartbeat Monitor

in this attachment file, what is the cut-off frequency of that circuit and what is the equation ? what if i can find some component of the circuit, what i must to do cause i can't find elco 0.66microF and some resistor. how i can design the circuit that i can find the component in my place?
tanx
 

Heartbeat Monitor

An electrolytic capacitor has a very wide tolerance. You must use accurate 2% or 5% film capacitors.

Nobody makes a 0.66uF capacitor. You make one with two 0.33uF film capacitors in parallel.
 

Heartbeat Monitor

how about the cut-off frequency of that circuit and what is the equation ? so i can make the value of componen like resistor and elco with my own.
tanx
 

Heartbeat Monitor

ranggaperdana_06114 said:
how about the cut-off frequency of that circuit and what is the equation ? so i can make the value of componen like resistor and elco with my own.
tanx
Click to expand...
Look up Sallen and Key highpass and lowpass filters in Google.

Don't use an electrolytic capacitor, its tolerance is poor. Use 2% or 5% film capacitors.

Multiply the capacitor value with the resistor value then keep the result the same using different values and the frequency will be the same.
 

Re: Heartbeat Monitor

how if i change the resistor 960k with 1M, 51K with 50K, and for the capacitor 0.66microF i change with 1microF? I use all of resistor wuth film type and the capacitor use the tantalum type. is the film capacitor same with ceramic capacitor?
tanx
 

Re: Heartbeat Monitor

A tantalum or ceramic capacitor is horrible for audio.
They will probably change the appearance of heartbeats on a 'scope.
Use film capacitors.

The difference between 960k and 1M is only 4% and you won't notice the difference.
The difference between 50k and a standard 51K is only 2% and you won't notice the difference.

The 0.66uF capacitor is actually two 0.33uF film capacitors in parallel. Don't use 1uF which will change the cutoff frequency and 0.5uF is not a standard value but two 1uF capacitors in series make 0.5uF.

You should also look at a Twin-t filter values.
 

Re: Heartbeat Monitor

how about my design about the filter LPF & HPF (see the attachment)?
 

Heartbeat Monitor

Most of the Sallen and Key circuits that have a Butterworth response and have gain have R1 and R2 with the same value.
Yours are different.
 

Heartbeat Monitor

how if i want to use frequency cut-off 1KHz, what is i must to do?
i've change the R1 and R2 = 3.2K.
 

Re: Heartbeat Monitor

ranggaperdana_06114 said:
how if i want to use frequency cut-off 1KHz, what is i must to do?
i've change the R1 and R2 = 3.2K.
Click to expand...
3.2k is not a standard resistor value.
You must look at Sallen and Key filters in Google. The articles have the formula for the parts for a certain cutoff frequency.

Don't make the resistor values less than 2k ohms or the opamps will not be able to drive them.
Don't make the capacitor values less than 100pF or stray capacitance will change the frequency.
 

Heartbeat Monitor

Can U explain the character of twin t notch filter?
why is the equation (fo=Center frequency) same with fc (cut-off frequency)?
in your attachment where is fo cause i don't see it?
 

Heartbeat Monitor

The twin-T filter has the center frequency defined in the formula. It doesn't have a cutoff frequency but insted it has a certain bandwidth and Q.
The twin-T filter can be included in the feedback loop of an opamp for a narrower bandwidth and a higher Q.
 

Re: Heartbeat Monitor

ranggaperdana_06114 said:
so where is the fo?
Click to expand...
The fo is the notch frequency of the twin-T filter. The resistor and capacitor ratios and formula are posted above with the schematic.
 

Heartbeat Monitor

the last thing i want to know, does anybodies ever has make the circuit of heartbeat monitor with that circuit i had post before (untitled.GIF) and this circuit works done?
tanx
 

Re: Heartbeat Monitor

i've found this circuit how about it?
LM 324 is not good what i use to change it?
tanx
 

Heartbeat Monitor

You should not use a noisy lousy old LM324 for this circuit with input resistors as high as 10M.

Your circuit does not have a balanced differential input so one input that is amplified will pickup lots of mains hum.
An low noise instrumentation amplifier IC should be used like is used on every other ECG circuit.

The diode feeding the transistor clamps the output to +1.4V.
The coupling capacitor will charge then not provide any more output unless something discharges it on each cycle.
 

Re: Heartbeat Monitor

in my attachment before (untitled.gif), what is the effect of R14 & R15, R4 & R5, R9 & R10 ? Cause i can't find some value of resistor in the market, what if i change it with the resistor that i can find in the market? what is the formula of that resistance?
tanx
 

Heartbeat Monitor

Keep the ratio of the resistors. In the filters if the ratio is too low then the response is droopy. If the ratio is too high then there will be a response peak near the cutoff frequency and the filter will oscillate if the ratio is more than 2.

Most resistors are common 5% values except the 960k can be 1M, the 9.6k could be 10k and the 5k could be 5.1k.

R9 and R10 set the gain of U2 to 9. The gain does not affect anything except the output level. instead of 16k and 2k, 33k and 3.9k could be used.
 

Status
Not open for further replies.

Similar threads

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Back
Top