Let’s start with some basic specifications from the manufacturer:

• 8MHz bandwidth
• 40MS/s sample rate
• adjustable trigger (level and slope) with single-shot mode
• voltage, current, resistance, capacitance and frequency measurement
• 10 screen memory
• 160×160 monochrome display with backlight
• USB connection

Specs seem quite good for a handheld device and it should accurately display signals up to 4MHz. Time to check it out!

Box full of goodies

This scopemeter comes in a black carrying case which is handy. Apart from the meter, the case contains four AA batteries of questionable quality, one set of test leads, set of clip-on alligator clips, BNC adapter, power adapter and a USB cable. Oh, I almost forgot – you also get a user manual and CD with mostly unusable software.

One of the obvious things that is missing is an oscilloscope probe which was left out of the package in order to keep the price low. I would prefer to have the probe instead of that power adapter. What I do like is the fact that the meter is powered with four AA batteries which should give it a decent battery life as opposed to using a single 9V battery (less capacity and more expensive than AA cells).

It

The meter looks great and build quality is really good. Case is plastic with a rubberish surround and no mater how you twist it there are no rattling/creaking sounds. It definately looks like it could withstand a fair amount of abuse before giving up and falling apart. One thing that could be improved is the plastic stand which bends a bit when you push the buttons while the meter is upright. There are also four bumps on the bottom which are supposed to keep the meter from sliding on the table but they don’t work well since the top two are plastic and have no real purpose. This is nothing to worry about since the meter will most likely be used in upright position where it is quite stable and slide-proof.

Speaking of user interface there is a nice big rotary switch for function selection, a couple of function keys and a big screen with backlight. As you can see from the pictures above, the screen is quite nice and you can adjust the brightness and contrast to suit your needs. Rotary switch makes a nice “click” sound when you rotate it and function keys are made out of rubber and don’t have any audible feedback (there is a beeper but only for continuity measurements). Speaking of function keys, first row is function-dependant and actual key functions are shown on the display. One last this to say is that the connector for external power supply is located on the left side near the bottom, protected with a rubber cover.

So far it looks like money well spent!

Work It

Now it’s time to say a few words about meter operation. You can use it in scope or multimeter mode and scope mode is available when measuring voltage, current or frequency. Other measurements such as resistance or capacitance are shown only in multimeter mode which is normal. Each time you select a measurement type via rotary switch the meter will go into “measurement type detection” thing where it decides wether it should use the scope or multimeter mode. There is also a dedicated MODE button which allows you to manually switch between modes. I found that measurement type detection a bit annoying because it lasts for a second or two (especially on frequency and capacitance measurements) for no apparent reason – it will always show the scope display when you switch to voltage, current or frequency measurement and I see no need for any sort of detection. It would be better if the meter would just remember the chosen mode for each measurement and revert to it when selected.

General responsiveness of the meter is…well…fine. There is a small delay each time you press a button but it’s not that bad. One thing that is bad and I mean really bad is the auto-ranging. It is slow as a turtle on sedatives! Seriously. It takes more than 10 seconds for the meter to find appropriate vertical and horizontal range. This goes for the multimeter mode as well – measuring resistors takes some time. Fortunately there is the manual mode so you can set the range and timebase yourself. There is also an option of moving the waveform across X and Y axes with a small inconsistency – in order to move the waveform up/down you use the F3 and F4 buttons when the RANGE function is active and to move it left/right you must use the F1 and F2 buttons when the TIME function is active (F3/F4 are used to adjust the timebase in this case). Not a big deal, you get used to it eventually. Maybe it’s one of those “it’s not a bug, it’s a feature” situations, who knows.

Ok, enough with that, here are some pics of the scope in action. First picture is what you get when meter is not connected to anything and second picture is a display of 220V 50Hz AC voltage. If you look more carefully you will notice how the “move” functions are flipped.

Following pictures are just some random shots of square wave signals at different frequencies. I don’t have any function generator (yet!) so I just poked around a finished PCB from another project looking for some test signals.

The meter also supports HOLD when in scope mode but apparently doesn’t like square wave signals at higher frequencies. That 2.55V and 4.347MHz readout is correct but the waveform should be a square not a sine.

As stated in the specifications there is an adjustable trigger whose level can be set to automatic or manual mode. Slope can be adjusted regardless of trigger mode. There is also a single-shot capture mode which is not quite so handy as it sounds since you cannot manipulate the captured waveform after acquisition (you can only scroll the screen horizontally by a limited range). It would be really nice if you could change the timebase of the captured waveform but it looks like the meter just takes a fixed number of samples required to fill two screens using current settings and that’s it. Nevertheless, it can be a very useful function.

What else is there?

OK, scope tested (more or less), time to tell something about other functions. HOLD mode is available in all measurements as well as REL function so you can easily filter out test leads’ resistance/capacitance. Multimeter mode works well apart from the terribly slow auto-ranging. Continuity test is resonably fast while capacitance measurement goes from fast to slow as capacity increases (around 20 seconds for a 100uF cap). I must say that I’m a bit puzzled as to why they have included a limited capacitance measurement since a temperature measurement would be more useful. You could for example set the sampling rate to 5 seconds and acquire temperature readings over 20 minutes.

As for other advanced functions…there are none. You can adjust display contrast and backlight as well as automatic turn-off timer and that’s it. Works fine for me. I should probably mention that when the meter automatically turns itself off it still drains the batteries, slowly but steady, so it’s a good idea to make sure that the rotary switch is in OFF position.

Final verdict

Overall, for an 8MHz and 40MS/s scope the performance is very good. Sine wave looks great even on higher frequencies while square wave is good up to approx. 1MHz if you are looking for an accurate waveform display. Recommended!

Thumbs up for: very good build quality and performance for the price, high input voltage range
Thumbs down for: auto-ranging

• adjustable time in 15-second increments (up to 10 minutes)
• lamp preheat feature
• two button operation (with audible feedback)
• countdown alarm (when timer reaches zero)
• relay for lamp switching
• 7-segment display

10 minutes?

I have chosen the upper time limit of 10 minutes which should be more than enough for most applications and initial time is set for a 5 minute countdown (I’m preparing a revised software which will load initial time from EEPROM so it can be configured manually). Step increment of 15 seconds has been chosen partially due to ease of operation because there are only two buttons – time adjustment and start/stop. Time adjustment is incremental meaning that each time you press the button time is incremented by 15 seconds. When upper limit is reached time is reset to zero and the process repeats. Start/stop button is used for, you will never guess, starting and stopping the timer with lamp preheat feature.

Serving it warm

Lamp preheat was added due to necessity – fluorescent lamps take some time to achieve full light output. It wouldn’t be a problem if it took exactly the same on every lamp turn-on, but that’s not the case as you might have guessed. What’s even worse is the fact that if you use multiple lamps with separate ballasts not all lamps will turn on at the exact moment. I have a situation where 2 out of 3 lamps turn on and the 3rd lamp still flickers for a couple of seconds then finally turns on. This kind of bugged me a little so I added the lamp preheat feature which basically turns on the lamps but doesn’t start the countdown (countdown is initiated by a second button press). If your lamps behave properly (incandescent or LEDs) you can simply double-click the start/stop button and it will skip the preheat function.

Bring it on

Countdown timer is separated into two boards – mainboard and I/O board. Mainboard PCB contains the power supply, lamp relay and PIC with its associated circuitry. I/O board contains the displays and buttons for user interface and the boards are connected via two 8-pin ribbon cables allowing you to place the I/O board where ever you want (i.e. outside the UV exposure box). Entire circuit can be powered from a small transformer with a secondary rating of 6-24V but I would recommend to keep the input voltage below 15V because of the voltage regulator IC.

As you can see from the schematics, entire circuit is quite common and simple. Displays are multiplexed meaning that they are driven one by one, really fast, so they seem to be constantly on due to persistence of vision. One nice feature with the 16F62x PICs is their internal RC oscillator which is accurate enough for this purpose and frees up two additional pins. As for the power supply, 7805 IC regulator seems to be all time favorite with DIYers and I’m no exception. You might want to fix it to a small heat sink especially if you are going to put the circuit next to the lamps in a closed box.

Use the relay to switch the live (or “hot”) AC wire!

Hi-res schematics and board layouts can be found at the end of this article.

Operation

Timer operation is nice and simple, just the way I like it! As mentioned before, you adjust the time with the TIME ADJ button and start or stop the timer with the START/STOP button. While adjusting the time the display will obviously show the time. When START/STOP button is pressed once the timer will go into lamp preheat mode and the display will alternate between “HEAT” and set time (you can still adjust the time if needed). On a second button press the timer will start the countdown and the display will show remaining time. When the countdown is complete the display will show a blinking “DONE” text (or rather “dOnE” on 7-segment displays) and an alarm will sound for indication which can be cancelled by pressing any button.

Flash before use

I have provided HEX files for 16F627(A) and 16F628(A) PICs so you can use whichever you have at hand. Configuration data is in the HEX file provided you select correct device before loading the file. In any case, all fuses should be off except power-up timer. Oscillator must be configured for IntRC with I/O functionality (since RA6 and RA7 are used for driving the buzzer and relay).

Give it to me

Here is the documentation available for this circuit. You can download individual items or an entire archive if you want to save a few clicks. Each PCB layout has a component view (bottom + top + silkscreen) to assist in component placement. Layouts are 600 DPI images so keep that in mind before printing (board dimensions are also available).

Hi-res schematic: Mainboard, I/O Board
PCB layout: Mainboard, I/O Board
Connection diagram: not available
HEX file: 16F627, 16F627A, 16F628, 16F628A
Software: not available
Complete archive: PIC Countdown Timer

Things to improve for Rev 2.0:
- remember last selected time and recall it on turn-on
- load configuration from PIC’s EEPROM (time limit, time step, audible button feedback, alarm and preheat bypass)

As you can probably notice the site is pretty much useless at the moment since I just started it.

What?

Here I will present my own electronics projects/circuits and provide all the schematics and instructions required to build them. You can also expect to find some random posts mostly related to electronics as well as component tests and small circuits which have extensive applications in other projects.

When?

As soon as possible! I already have some cool projects for you, just be patient until I write something meaningful.

For whom?

Mostly for semi-experienced people interested in electronic and DIY. I will assume you know some basic theory and have skills necessary to etch your own PCBs and solder components in place.

How?

In plain english and without much fuss! Expect some gramatical and spelling errors since english is not my native language.

“See” you soon!

Arcom