Wednesday, January 18, 2017

Room Temperature Monitor and Alarm

I am required to design an electronic system that monitors the temperature between(22-24 degrees Celsius). Given are power supply, Analogue subsystems and digital subsystems. the accuracy of the digital control circuit is limited by the accuracy of the temperature sensors. A green LED is lit to indicate correct temp range.

Room Temperature Monitor and Alarm

A red LED indicates out of range temperature and in sustained events( where this indicates out of range temp which is counted, displayed and timed) a large LED is lit. In a event where the sustained event exceeds 6 days an alarm is sounded to indicate a possibility of a system failure.

Now I need help on how to come up with the correct circuit diagrams which can work , how to simulate and to get the project done, please help, even direct me to books on the project or other sites please...

Mail from AM South Africa

First acquire the room temperature as an analog value. This requires a sensor for the range you indicated a Semiconductor sensor like LM335 will do fine.

Use the ideas on this page - Circuit Design Service and Projects 

Now monitor and generate Logic outputs for temperatures out of range using a Window Comparator. Then for the timed events you need a Timer circuit. And a Logic to set off the Alarm or Annunciator.

All this can also be done by a PIC circuit if you can write some code. I have given some pages to see, study them, that will be sufficient

Tuesday, January 17, 2017

RTD and Connections - Mini 3W 4-20mA transmiter

I´ve been looking at the mini-three wire 4-20mA transmitter and it is everything I need to try a new way of measuring the temperature. It is great but there are a few questions about it I hope you can answer them:

- What is the meaning of RYIN and RXIN?

- You say that it is a three wire circuit but I can only seen two points of connection for the PT100. Is it because the signal conditioner of the three wire PT100 isn´t drawing?

- The OUTPUT connector is where I can find the 4-20 mA?

I hope there is no problem to answer these quetions to me, I would be really pleased if you can do it and I´d appreciate it.

Many thanks and hope to hear soon from you.

Mail from AD - Spain

All the transmitter circuits can be seen here. Industrial Process Control Circuits

RTD and Connections - Mini 3W 4-20mA transmiter

As this circuit was popular i have made a separate page for it. Mini RTD Pt-100 Three Wire Transmitter

A ex-customer of mine, (ex because i don't manufacture now.) wanted a low cost transmitter, so i made this.

What is the meaning of RYIN and RXIN? - Connect it to any two wire RTD

For RTD read more RTD Omega - RTD Intro

You say that it is a three wire circuit ....

+V .... -V .....OUTPUT ...... on the left are the three wires.
+V and -V is the Supply. OUTPUT is a Current Sink Directly Proportional to Temperature on RTD.

In the Costlier 2 Wire systems, Current Drawn by Transmitter is Directly Proportional to Temperature on RTD. Two wire systems can be just looped, 3 Wire like mine has to be wired, more wire.

The OUTPUT actually drinks 4-20mA prop to Temp. Put a Ammeter (fused) in 0.2A range in Series with a 50 Ohm Resistor. Black Lead of ammeter to this OUTPUT. Red Lead to one end of 50 Ohm. The other End of 50 E to +V.


Thank you for your quick answer. I think I´ve got it. Although there is another question that has come up to me while reading your email. +V and -V is the supply I apply to the PT100? I am applying 24DCV so I suppose the +V is the 24V and -V is the ground, isn´t it?

Reply from AD

YES - +V is the 24V and -V is the ground, anything above 12V will work.

The 24 V should be applied to +V and -V only. Not to RTD Directly. RTD Directly Connected to 24V will BURN the RTD.

RTD is a Thin Platinum Film or Wire of 100 Ohms worth. The Resistance varies with temperature.

A 1mA ONLY Constant Current is sent thru the RTD to measure the Voltage across it. From the measured voltage 100mV for the 100 Ohms is subtracted using opamps.

The balance mV is Directly Proportional to Temperature. This mV is converted to 4-20 mA. 4-20 mA is a Strong Signal for Current Loop Transmission.

4mA is 0% 12mA is 50% and 20mA is 100% of the range you calibrated.

Related Reading

Monday, January 09, 2017

Comparison of Thermocouple and RTD

message: what is the pros and cons of rtd,over thermocouple

Mail from MU

Read the pages in links below, that ought to clarify your doubts. Last edited February 16, 2007

Comparison of Thermocouple and RTD

Comparison of Thermistors, Thermocouples and RTD's ... Unlike RTD's and thermistors thermocouples have plus and minus legs so polarity must be observed.
Comparision of Thermistors, Thermocouples and RTD's

Thermocouple - No resistance lead wire problems, Fastest response;  RTD - Most stable, accurate, Contamination resistant,; Thermistor - High output, fast, Point temperature sensing.
Watlow - Temperature Sensor Comparison Guide

Since thermocouples measure wide temperature ranges and are relatively rugged, they are very often used industrial and process applications where accuracy may be a less important factor.
RTD vs. Thermocouple comparison chart


Thursday, November 17, 2016

Temperature control for CPU

My name is IS and I am looking for somebody to design and build a simple circuit that controls a small computer fan using either a 120V plugin or a battery using a temperature switch...

In other words I would give you the set point and at that temperature it would turn on the fan and when it drops below the set point it turns off the fan.. the temperature would have to be measured using a contact type measuring device like a thermocouple or thermistor.

Mail from IS Brazil

Go thru these links and visit all pages -
 Try this One Chip solution from Maxim Semiconductor

MAX6641 temperature sensor and fan controller

MAX6641 temperature sensor and fan controller

"Accurately measures the temperature of its own die and the temperature of a remote pn junction. The device reports temperature values in digital form using a 2-wire serial interface. The remote pn junction is typically the emitter-base junction of a common-collector pnp on a CPU, FPGA, or ASIC."

Some of my circuits are here Industrial Process Control Circuits

    Wednesday, November 16, 2016

    Decade Digital Counter Module - LED and LCD

    I'm looking for a source of decade counters and wonder if you can help? I don't want to build the counter, I want to buy a functioning counter module.

    Do you have source suggestions?

    Mail from SF USA On 1/1/2011 1:34 AM

    Try These, some are kits, some ready modules.

    delabs has a resource on counters -   Digital Timers Counters and Clocks

    Easy way to build a counter since the 80s is using ICM7217.

    4-Digit, Presettable, LED Up/Down Counter

    4 Digit Counter Module - The counter module is supplied pre-assembled and tested and provides the building block for counter and timer circuits. 

    The counter module may also be used as an accurate digital timer with the addition of a suitable timebase and gating circuit.

    4017B Decade counter (5-stage Johnson counter) - Learn about the 4017 by building the DOCTRONICS Light Chaser Project:

    Multifunction Counter-Timer Electronic Kit K8035 - Counter-timer K8035 is a multifunction electronic kit that is handy for all sorts of counting and timing applications. Counts up or down from your own pre-defined starting point. Counter-timer can count each time you press the onboard push buttons, or count externally supplied voltage pulses.

    VM147 - Panel Counter MODULE - offers a very practical solution for counting parts, people, scorekeeping, etc.

    LCD counter module 7016 6-digit - Snap-fit module with 4-digit LCD and its own power supply (not included). This counter module is therefore suited to PCB-mounting (snap-in frame can be removed) as well as front-panel mounting

    Try Radio Shack ask for - LCD Counter Module - it may be expensive compared to above.

    Tuesday, November 15, 2016

    Three Phase AC SSR for 1 HP Motor

    I need to build a three phase AC(in 120V)-AC(out 440V) solid state relay to turn on or off a small(1hp) three phase AC motor. Do you have any idea?

    Can I use three MOC3043 and three TIC236E triacs of course with all opto LEDs in series?

    Mail from FJP Brazil

    See my SSR Circuits , Solid State Relays or SSR Choose a DC Control and AC Load SSR, That is what you seem to indicate.

    1 hp = 745.699872 watts

    Let triac have 10 A and 600V AC ratings, BTA12600 But now BTA12600 is obsolete, adapt to what is available in your country.

    MOC3043 is fine, isolation must be good, take care. Use a Heatsink.
    ON-OFF Control inputs can be parallel not series.

    Here is a picture of a Solid State Contacter from Power IO

    Three Phase AC SSR for 1 HP Motor

    On this page you can see a 3 phase SSR i built. Three-Phase-SSR.


    Tuesday, October 11, 2016

    Isolated RS-232 Serial Port Interface to SBC

    Subject: Help! - Date: Thursday, December 16, 2010, 4:40 PM
    Hello delabs
    I have question about one your project . The question is about the project RS232 with Opto-Isolation on I need this very much . Do i need to use max 232 or something else on uC side , or i connect directly on uc rxd/txd pins .I need this answer so much , im new in this . please help

    Best Regards
    Mail from SS (CC) - Russia

    This was done when MAX232 was not invented yet. This interfaces with PC well. If you want to make a PC based datalogger with 80C31, then this gives a safe PC to 80C51 SBC Serial Link.

    Suppose a Temperature sensor or a Heater Open up. If the High Voltage reaches the A/D and SBC. The PC will be next Victim.

    The Affordability and Product Safety Aspects, Including Fire Hazards; made me use this, as any other solution would make product expensive. An Opto-coupler and well designed PCB can give more than 2 KV isolation.

    The MAX232 does not have isolation, but look here Isolated RS-232 Transceivers. The MAX232 also generates a Higher Voltage level, like 10V for proper RS232.


    The MAX3250 is a 3.0V to 5.5V powered, ±50V isolated EIA/TIA-232 and V.28/V.24 communications interface with high data-rate capabilities.

    The circuit i have made is very old and you can interface any serial uC with PC. No other chip is needed. It is cumbersome to build it and get it working for many. I suggest use one of these chips from Maxim unless you want to Adventure like me.