Tuesday, December 16, 2014

Sound activated RF remote relay

I am to build a circuit that operates off some kind of sound input into a sound activated circuit, preferably some kind of voice operated relay circuit. This relay is then to operate some kind of signal transmitter such as RF which activates a signal receiver and outputs onto a buzzer and an LED. It is to work something like a baby monitor, but in quite a simplified fashion, nothing too complex. I would be interested in circuit diagrams and related documents.

mail from CK

What is the max. distance between transmitter and receiver. Is it to monitor if the baby has woken up?

Voice Activated FM Transmitter - Alltronics

Voice Activated FM Transmitter - Alltronics

A sound activated switch and an FM transmitter joined together to make a voice activated FM transmitter. Set the level of sound that it will respond to. Two stage transmitter needs tuning to get best results. Tuning Kit and instructions enclosed. Transmits up to 1 km in the open depending on the aerial used. ZTX320 transistor specification.


The max distance between transmitter and receiver i would say would be no more than 20 meters or so. It should be designed to activate the transmitter when the vox detects noise up to 5m away, which would the trigger the remote alert device.

Look at a Solution here from TI (2014)

Baby Monitor TI App

Baby Monitor TI App

Baby Monitor is a capture and display system used to remotely watch infants. At the capture side, a microphone and a camera are placed near the child to transmit audio and video to the care giver. A parent talk-back audio system is also provided where baby can listen to the voice of caregiver.

A baby monitor is a video-based product used to see and hear an infant when the caregiver is physically not present near him/her.

I have already looked at a number of schematics and circuits i have found through my research on the internet and at my local library. I have already seen the circuit for the link that you provided, (Voice Activated FM Transmitter) and that would be ideal for the first part. I just need to know now what i would need for the receiver and remote alert unit: schematics, etc, and whether the receiver unit component values are dependant upon those of the transmitter. (All the receivers schematics I have found up to now are designed to receive speech and music like a radio.... I want to use the receiver to activate a buzzer and LED, as a remote alert device, I suppose this could also be used as a noise activated intruder alert system too!)

mail from CK

I am still trying to understand your requirement. 20 mts is quite small and is easy. What is the frequency range you need for min interference with your other gadgets. Also what are the parts your are looking at, do you need very low cost solution ? Is it just for your home use or is it for production ? will a toy circuit do ?

If you want a modern circuit, the best way is to use ready made modules. these may work at 30-200 Mhz range, some are AM some are FM.

The microphone goes to an amp which turns on one bit of the transmitter. The receiver decoded output can drive a buzzer, relay or LED as chosen.


20 meters is an adequate distance as generally the circuit is intended to be used around the house while the child is upstairs sleeping, so 20 meters should cover that quite well.

I have no other gadgets operating on radio frequencies within my home, so that should not really be a problem, i don't think.

I would prefer, if possible, an economic solution to this, a really simple circuit would do fine, however I don't want to purchase a 'hobby' kit to do this, I would prefer the circuit diagrams so I can see what parts for it i would need to get for myself...that is not a problem.

mail from CK

You must be able to study, build and test them and troubleshoot if required. i can just guide you to put things together, RF is delicate and difficult work. remember after all the efforts you put, it may not work, if you are not persistent.


Sunday, December 14, 2014

Analog Ground and Digital Ground

I need a few tips from you on Analog design in general: I have seen that a lot of ADCs have a separate AGND and DGND, but they are shorted at the chip. I believe this is required to keep the noisy digital return paths sperate from the analog return paths.

However if both get shorted at the ADC what purpose will it serve? Do you know any good application notes which clarify this ?

mail from TF

Some ADCs have differential inputs which are some times not used. Strain gauge bridge amplifiers may need differential measurement. When common mode measurements are made one end can be grounded.

Compatible Electronics, Inc - Specialize in EMC testing for FCC as well as EMC / EMI testing for CE Mark compliance under the Electromagnetic Compatibility Directive.

Grounding Demystified - Seminar of Compatible Electronics Inc

Grounding Demystified - Seminar of Compatible Electronics, Inc

In Other ADCs the analog ground and digital ground may be shown shorted in circuit.

The digital ground carries the power current to digital circuits in the chip. This current will have pulses due to switching of many digital outputs. The short, wire or link shown is circuit is assumed to be ideal in design. In fact the short, wire or PCB track has resistance, capacitance and inductance. These small values of RLC in a PCB track can cause strange AF-RF behavior.

Staying Well Grounded Analog Devices

Successful PCB Grounding with Mixed-Signal Chips Maxim Integrated

The ideal is the digital ground should be a track going directly to supply ground. The analog ground should be a track linking to the measurement circuits or sensors. The analog and digital ground now should be linked close to chip. This link will carry no current, so no ground loop, so no lifting of ground.

This ground problem can reset microcontrollers or timers when a solenoid or relay operates. Similar problem produces hum, feedback oscillations in Audio systems.

Radiating grounds, ground planes, thick tracks, decoupling caps can be remedies. see this link too.

Noise and Hum in Audio Circuits


Saturday, December 13, 2014

Millivolt Source for Calibration

I was looking for a simple MILLIVOLT SOURCE for ion electrode testing and came across your pdf schematic. This design would appear to fit my needs perfectly, but I have a couple minor questions that I hope you could provide answers for:

The pdf drawing (Document Number ST02) is not clear with respect to the U4 op amp circuit, namely the connection of components R9, P4, and R10 to the op amp. I assume they connect to U4 pin 1 and pin 8 respectively as a balance or offset feature -- maybe you can confirm this.

Also, it is not shown where the terminals of the potentiometer P4 connect to -- do they go to Vcc and Vdd? And where does the slider 2 connect?

There is no problem with the rest of the circuit being readable. I expect capacitor C7 would have to be a low leakage type.

NPN transistor Q1 is shown as a BELL100, which I could not find in my catalogues. Could you suggest a suitable replacement?

Thank you for taking the time to check these points. Your answers will help in bread boarding this instrument

mail from AS

R9, P4 and R10 are for balance and offset as you said you can use it that way, but see the new circuit.

Millivolt Source for Calibration

Millivolt Source In this link see at bottom this circuit millivolt source, pdf.

I have put a better offset null, OP07 has around 75uV offset error which may show as +/- 1 count error on 4 1/2 DPM 19999 counts. You can skip it if you are using a 3 1/2 digit DPM as the error will not show, even it 4 1/2 it may be upto 2 counts only.

C7 can be a low leakage plastic cap, even a tantalum electrolytic is ok, aluminum electrolytic may cause a very small error.

Q1 can be any npn that can take 100mA current, do not use RF devices, 2N2222 is best.

If you use a DPM protect DPM inputs with clamping diodes or zeners or an error in bread-boarding may send +/- 12V to DPM and it may be damaged. Some DPMs come with protection like DMMs. use the circuit in del2003.pdf in analog section to make a 4 1/2 DPM.

Also in 2000mV range do not short outputs as the Q1 may get damaged, and in 200mV and 20mV range the output impedance is 10 ohms which is good for calibrating any high input impedance instrumentation like a process indicator etc. loading with 100K 10K will cause error.
Most instruments are very high impedance so it is fine.